Email Migration Guide

Import PST and EML into MimecastThe pre-upload checklist for Legacy Archive Data Management

Import PST and EML into Mimecast Legacy Archive without rejects: accepted formats, 5 to 10 GB sizes, per-user naming, EJF journals and Bad Addresses.

By 📅 Updated: September 7, 2026 ⏲️ 11 min read ✓ Verified against Microsoft's published documentation 🖼️ 4 screens

Mimecast Legacy Archive Data Management accepts two things: PST files and zipped EML files. Hold each PST at 5 to 10 GB (50 GB is the ceiling), each EML ZIP under 15 GB with 2 GB recommended, no passwords and no more than 60,000 items per folder. Name per-user PST files after the mailbox address and put journal data in the NotPerUser folders as SJF or EJF. Check every sender and recipient against your Mimecast accounts first so nothing comes back as a Bad Address. MSG folders, loose EML files and oversized PSTs are the three things that get sent back, and Univik Email Archive Converter fixes all three in one run.

To import PST to Mimecast, you prepare the files and Mimecast's team ingests them. There is no upload button. Anything that breaks their rules comes back to you with a report and a delay. Most of those rules are simple once written down, and the point of this checklist is to write them down. It covers what the service accepts, how big the files can be and how they must be named and organised. It also covers journal data and how to avoid the Bad Addresses report that stalls so many first attempts. Follow it and the ten working day review is the only wait.

Before You Prepare the Mimecast Import

Time1 to 3 days prep
DifficultyMedium
CostMimecast PS quote
Runs onWindows + your archive

What is the data you are sending? The answer decides most of the checklist.

Per-user mailboxes from an old archive or PST collection Enterprise Vault, SourceOne, a share full of leaver PSTs. Each file must be named after the mailbox and sit in the PerUser folder. Prepare the files → A journal archive from Exchange or a compliance platform Envelope journals go in as EJF, plain copies as SJF, both in the NotPerUser folders. The BCC data is only kept if the envelope is. Read the journal rules → An eDiscovery export in MSG or loose EML Arctera, Purview and Proofpoint hand you formats Mimecast will not take. Convert to PST or zipped EML before anything else. See what is accepted →
Gather these before the first file is prepared
  • Your Mimecast account list with aliasesPer-user data is matched to existing Mimecast accounts by SMTP address. Export the Directories list so you can reconcile against it.
  • The ingestion cut-off dateMimecast asks for the date after which live archiving took over. Everything you send should be older than it, or you get duplicates on the timeline.
  • A company name and asset tag for the root folderThe transfer expects a root such as Contoso_MC2026 with Pst and Eml under it. Decide the name now so every prepared file lands in the right place.
  • Disk space equal to twice the sourcePrepared files sit next to the originals until Mimecast confirms the import. Keep the originals until then; a resubmission needs them.
Six terms the Mimecast team will use
Legacy Archive Data Management
Mimecast's service for ingesting historical mail from PST files and EML archives into an account. It used to be called Simply Migrate.
Per-user (PU)
Data that belongs to one mailbox. It is written into that user's Mimecast archive, so the account must exist and the file must carry the address.
Not per-user (NPU)
Journal or shared data that is not tied to one mailbox. It is indexed by the addresses inside each message instead.
SJF
Standard Journal Format. An exact copy of each message with no envelope, so BCC recipients and distribution list members are not recorded.
EJF
Envelope Journal Format. Each message wrapped in a journal report that lists every real sender and recipient, BCC and expanded lists included.
Bad Addresses
Mimecast's report of per-user files whose address matches no account. Nothing in that file is imported until you fix the address or create the account.

Prepare PST and EML Files for Mimecast in 5 Steps

The order matters for a Mimecast import. Sorting first tells you which rules apply to each file, and reconciling last catches the addresses that would otherwise come back as rejects.

1

Sort per-user mail from journal and shared data

Make two piles. Mail that belongs to one mailbox, including leavers and shared mailboxes that exist as Mimecast accounts, is per-user. Journal archives, compliance captures and anything with no single owner is not per-user. Everything downstream, from naming to folder placement to how Mimecast indexes it, follows from this split. Write the pile against each source file in a sheet; you will need that sheet again in Step 5.

You should seetwo lists. If a file mixes both, such as a PST that holds one user's mail plus a journal folder, it has to be separated before Mimecast sees it.

2

Convert anything that is not PST or zipped EML

Mimecast takes PST and zipped EML. It does not take MSG files, loose EML files, MBOX, NSF or ZIP files that contain PST. That rules out a Purview eDiscovery export in .msg form, an Arctera MSG with EDRM export, a Thunderbird profile and most forensic outputs as they stand. Convert them to Unicode PST with one file set per mailbox for per-user data, and keep the folder structure. Univik Email Archive Converter reads MSG, EML, MBOX and the archive platforms' own export packages. It writes PST in the shape the next two steps need. The MSG to PST and EML to PST guides cover those two formats in detail.

Univik Email Archive Converter · Converter output settingsScreen 1 of 4
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Step 2PST output, 8 GB parts, address naming, passwords removed, envelopes kept. One run covers Steps 2 to 4.

3

Split every PST file to size and remove passwords

Per-user PST files should be 5 to 10 GB. Mimecast accepts up to 50 GB when the file is healthy and opens in Outlook, but anything over 20 GB goes into extended processing and slows the whole batch. Any single item over 512 MB is dropped. Password protected PST files and ZIP files are rejected outright, and so are ZIP files of PST. For EML, zip the files per user with 2 GB as the target and 15 GB as the hard maximum, plain .zip only, Zip64 is fine. No folder may hold more than 60,000 items in either format. Files of 1.96 to 2 GB are a red flag: they are usually ANSI files at the wall and must be proven to open before you send them. The split a large PST file guide has the Outlook method for a handful of files; a converter with a split setting handles hundreds.

4

Name the PST files and build the Mimecast folder tree

A per-user PST takes the mailbox address as its name: priya.adams@contoso.com.pst, then priya.adams@contoso.com.1.pst, .2.pst and so on for the parts. Not per-user files can carry any name as long as the extension is there. The folder tree under your root is fixed: Pst\PerUser, Pst\NotPerUser_SJF, Pst\NotPerUser_EJF, and the same three under Eml, where per-user EML ZIPs sit in a subfolder per address. Nothing else goes in the tree. Manifests and your own logs travel separately.

Windows Explorer · PerUser folderScreen 2 of 4
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Step 4The PerUser folder. Every file carries the mailbox address, parts numbered, all under 10 GB.

Windows Explorer · NotPerUser_EJF folderScreen 3 of 4
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Step 4The NotPerUser_EJF folder. Journal parts by quarter, any name, envelopes intact.

5

Reconcile addresses against Mimecast accounts

Every per-user file in a Mimecast PST import is matched to an account by the address in its name. Leavers who were never given an account, old aliases, renamed domains and shared mailboxes that only exist in Exchange all come back as Bad Addresses. The file waits until you answer. So check first. Export the Directories list from the Administration Console and compare it with your sheet from Step 1. Resolve each miss: add the alias or create the account, or move the file to a NotPerUser folder if nobody owns it. Addresses inside the messages that are not Mimecast internal domains are treated as external, which is fine. Per-user file names, though, must match an account exactly.

You should seezero unmatched addresses in the per-user list before the transfer. Every one you fix here saves a round trip of days later.

Excel · Address reconciliationScreen 4 of 4
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Step 5Reconcile before you send. Two Bad Addresses and one alias caught here would have stalled the batch.

What Mimecast Accepts: Formats and Size Limits

RulePSTZipped EMLNot accepted
Recommended size5 to 10 GB per file2 GB per ZIPMSG files
Loose EML outside a ZIP
ZIP files containing PST
MBOX, NSF, EDB
Password protected PST or ZIP
Tasks, notes, journal items, stubs from other archives
Maximum50 GB, over 20 GB slows15 GB, .zip only
Items per folder60,00060,000
Single itemUnder 512 MBUnder 512 MB
Per-user namingaddress.pst, address.1.pstPerUser\address\*.zip
DuplicatesAllowed. Mimecast deduplicates on Message-ID plus sent date, so overlapping exports are safe.

Item types that import are mail, delivery and read reports and calendar items. Contacts, tasks and sticky notes are skipped. Retention is applied from the sent date of each message, so a message from 2014 sent into a seven year retention account will not be visible; check the retention period before you spend days preparing old data that will not surface.

Per-User, SJF and EJF: Putting Journal Data in the Right Folder

Journal data is where imports go wrong quietly. The files are accepted and the batch completes. Months later a legal search finds no BCC recipients, because the journal was sent as plain copies. Mimecast's two not per-user folders exist to stop that.

NotPerUser_SJF
Standard journal format: an exact copy of each message. Fine for a shared archive or a journal that was never envelope journaled. BCC and distribution list members are not recorded, and Mimecast may need to run recipient resolution, which extends the timeline.
NotPerUser_EJF
Envelope journal format: each message inside an Exchange journal report that lists every real sender and recipient. This is what an Enterprise Vault or Exchange journal archive should become. Keep the envelope through any conversion.

A per-user file that turns out to hold a journal is the worst case, because the whole thing is attributed to one account. Sort it out in Step 1. The journal mailbox migration guide goes deeper on how envelope journals are structured and how to export them intact from Exchange, Enterprise Vault and Purview.

The Mimecast Hand-Off: What Happens After You Are Ready

  1. Order and scopeLegacy Archive Data Management is a Professional Services engagement priced by volume, with a minimum order. Your account manager scopes it and the ingestion team sends a data collection form.
  2. Information within ten working daysMimecast asks for the cut-off date, the account list, the data types and the folder layout. Answer inside ten working days or the request lapses and goes back into the queue.
  3. TransferThe team provides the transfer method and media details for your data. Send the prepared root folder as is, with nothing extra inside it.
  4. Ingestion and the Bad Addresses reportFiles are processed in the order received. Per-user files with no matching account are listed in the Bad Addresses report for you to correct. Inaccessible PST files can be resubmitted twice.
  5. Review and sign-offYou check the archive, and the import is deemed complete after your confirmation or after ten working days. Keep the originals until this point.
Prepare the whole archive for Mimecast in one converter run

Univik Email Archive Converter reads Enterprise Vault, Arctera, Purview, Proofpoint, MSG, EML and PST sources, writes Unicode PST parts at whatever size you set, names them after the mailbox, strips passwords and keeps envelope journals intact. Runs offline on your own hardware.

⬇️ Download Free Trial See All Features →

Windows Server or Windows 11 / 10 · Outlook not needed · the free trial prepares a sample per mailbox so you can check the naming and sizes first

After the Mimecast Import

The Mimecast PST import is done when
  • A search in the Administration Console for a 2015 message returns it under the right user with its attachment, and a journal search shows BCC recipients from an EJF file
  • The Bad Addresses report is empty or every entry has been resolved and resubmitted
  • Your reconciliation sheet shows an account against every per-user file and a folder against every journal part, and Mimecast has confirmed completion

Then set retention and legal hold as the compliance team requires; expired segments are purged after 90 days unless a legal hold is active. Keep the prepared files and your per-email conversion logs for the period your policy sets; together they are the chain of custody for the import. If the source of all this was an Arctera or Veritas export, the full story from that side is in how to export Arctera eDiscovery email to PST. If Mimecast is the platform you are leaving rather than joining, read leaving Mimecast: how to export the archive in time instead.

Fix Common Mimecast Import Rejects

Three rejects account for most Mimecast import delays. The red box is what the ingestion team reports and the green box is your answer.

1Bad Addresses report lists dozens of per-user files

Do this: The file names carry addresses with no Mimecast account: leavers, old aliases, a renamed domain. Create the accounts or add the aliases in Directories, rename any file that used a display name instead of an address, and resubmit. Files nobody owns move to NotPerUser_SJF.

2PST reported as inaccessible or password protected

Do this: The file is damaged, is ANSI at 2 GB or has a password. Run ScanPST, or rewrite it with a converter that reads the content directly and writes a fresh Unicode part without a password. You get two resubmissions per file, so fix it properly the first time.

3Journal data imported but searches show no BCC recipients

Do this: The journal went in as SJF or the envelope was stripped during conversion. Re-export from the source in envelope form, convert with the journal report kept intact and resubmit to NotPerUser_EJF.

Preparing a Mimecast ingestion and unsure whether your export will pass? Zip ten files from one mailbox plus one journal item and pass them to our support team along with your Mimecast cut-off date. We will tell you what needs converting before you commit.

Mimecast import questions

What file formats does Mimecast accept for a legacy archive import?
PST files and zipped EML files only. PST can hold standard per-user mail or SJF and EJF journal messages; EML ZIPs can hold RFC 822 messages or EJF. MSG files, loose EML, MBOX, NSF and ZIP files that contain PST are not accepted and have to be converted first.
What PST size does Mimecast accept?
The recommended range is 5 to 10 GB per PST; up to 50 GB is accepted if the file is not corrupt and opens in Outlook. Files over 20 GB go into extended processing. Single items must be under 512 MB and no folder may hold more than 60,000 items.
How should I name PST files for a Mimecast per-user import?
After the mailbox address: priya.adams@contoso.com.pst for the first file, then priya.adams@contoso.com.1.pst and so on for additional parts. Not per-user files such as journals can have any name as long as the .pst extension is present.
What is the difference between SJF and EJF in Mimecast?
SJF, Standard Journal Format, is an exact copy of each message with no envelope, so BCC recipients and distribution list members are not recorded. EJF, Envelope Journal Format, wraps each message in a journal report listing every real sender and recipient. Journal archives from Exchange or Enterprise Vault should be sent as EJF.
What does the Mimecast Bad Addresses report mean?
Per-user files were named with addresses that match no Mimecast account. Nothing in those files is imported until you create the account, add the alias or supply a corrected address. Reconciling your address list against Directories before the transfer avoids the report.
Does Mimecast accept password protected PST files?
No. Password protected PST files and ZIP files are rejected and reported back to you. Remove the password in Outlook or rewrite the file with a converter before you include it, and never zip a PST.
About This Guide

Written and maintained by and the Univik team, developers of Windows data conversion and recovery software since 2013. The formats, size limits, naming rules and process steps in this checklist were checked against Mimecast's current Legacy Archive Data Management documentation. Last verified September 7, 2026. Need a hand? Contact our support team.

Sources checked: Mimecast, Governance and Compliance: Legacy Archive Data Management · Mimecast, Archiving PST Data · Mimecast, Archiving EML Data · Mimecast, Legacy Archive Data Management prerequisites · Mimecast, Bad Addresses

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