Email Migration

How to Migrate Zimbra to Office 365 Step by Step

Leena Taylor Paul By Updated September 7, 2026 10 min read
Quick Answer

To migrate Zimbra to Office 365, either export each mailbox as a .tgz and bring it into Microsoft 365 as a PST, or let a tool sign in and write the mail straight to the target mailbox. The Univik Zimbra Converter does both and keeps folders, dates, attachments, contacts and calendar. For a whole live server, an IMAP batch in the Exchange admin center moves the mail instead.

Moving off Zimbra usually means the mail has to land in Microsoft 365, where the rest of the business already works. There is no single button for it, and the right path depends on where you start: a stack of exported .tgz files, or a Zimbra server still running that you want moved to the cloud. This guide walks the three routes to migrate Zimbra to Office 365. It covers what each one carries and how to check the result.

Why Teams Migrate Zimbra to Office 365

A Zimbra to Office 365 migration nearly always starts as a business decision rather than a technical one. Running Zimbra Collaboration on your own hardware costs staff hours and a support contract, while Microsoft 365 combines mail, calendar and the rest of the office suite into one subscription. When the rest of the company lives in Outlook and Teams, the mail is the last thing left on the old platform. Compliance and search also push the move, since Microsoft 365 puts every mailbox behind one retention and eDiscovery setup. Whatever the driver, the old Zimbra mail still has to arrive intact.

The Three Ways to Move Zimbra to Microsoft 365

Three routes cover almost every case. The first two work from an export, so they need no live server and they carry the whole mailbox. The third reads a running server over IMAP and suits a large setup that has to stay online during the switch-over. Pick by your starting point rather than by which sounds fastest.

1. Export then import a PST
Best when the server is off or you have .tgz files. Brings mail, folders, dates, contacts and calendar.
2. Send straight to a mailbox
The tool signs in to Microsoft 365 and writes the export into the target mailbox in one step.
3. IMAP batch, live server
Runs from the Exchange admin center against the running server. Moves mail folders, not contacts or calendar.
Three routes from Zimbra to Office 365, each suited to a different starting point

Export the Zimbra Mailbox and Import It to Office 365

This is the safe default when a server is being retired or you were handed a folder of .tgz files. First you export Zimbra mail to .tgz if you do not already have it, then turn each file into an Outlook PST and import Zimbra data into the matching Microsoft 365 mailbox. The Univik Zimbra Converter reads the .tgz and writes a clean PST that already holds the folders, dates, attachments, contacts and calendar. It can also write EML or MBOX when another system needs those instead. Our companion guide on how to convert Zimbra to PST covers that first step in full.

  1. Convert the export to PSTLoad the .tgz into the converter and write an Outlook PST for each mailbox.
  2. Make sure the 365 mailbox existsCreate and licence the user in the Microsoft 365 admin center so the mail has somewhere to go.
  3. Bring the PST into Microsoft 365Use Network Upload in the Microsoft Purview import service, or open the PST in an Outlook that is signed in to the 365 account.
  4. Check the countsCompare the folder counts in 365 with the source so you know every message arrived.
Univik Zimbra Converter start screen with Open, Review, Export and Extract steps for moving a mailbox to Office 365
The converter opens on a plain Open, Review, Export and Extract flow

For the upload step itself, our walk-through on how to import a PST to Office 365 covers the Network Upload and drive routes with the exact screens. The PST path is the one that also brings the address book and the calendar, which the live IMAP route cannot.

Send Zimbra Mail Straight to Office 365

The converter can skip the PST middle step. Point it at the export, choose Office 365 as the target, sign in to the tenant and it writes the mail into the mailbox for you. This suits a small number of accounts where you would rather not keep PST files on disk. Folders, dates and attachments arrive the same way. Contacts and the calendar come across too.

Univik Zimbra Converter export menu with Office 365 as a target, alongside PST, EML, MBOX and Gmail, for a Zimbra to Office 365 migration
The Export menu offers Office 365 as a direct target, next to PST for the import route
  1. Open the exportLoad the .tgz file, or sign in to read the live Zimbra account.
  2. Choose Office 365 as the outputPick Office 365 in the Export menu and approve the sign-in to your tenant.
  3. Map to the mailbox and runPoint it at the licensed target mailbox and start the transfer.
Loading a Zimbra .tgz export in Univik Zimbra Converter before sending the mail to Office 365
Point the tool at the exported .tgz, or sign in to read a live account

IMAP Migration for a Live Zimbra Server

When the Zimbra server is up and you want the cloud to pull the mail across, an IMAP migration is the built-in path. It runs from the Exchange admin center in Microsoft 365 and copies the folder mail on its own, so people keep working while it runs. First turn on IMAP in the Zimbra admin console so the server answers on the secure IMAP port 993. Then build a CSV that lists each mailbox on its own line, with the email address, the user name and the password in three columns. One file can hold up to 50,000 rows and 10 MB.

  1. Turn on IMAP in ZimbraEnable IMAP for the accounts in the Zimbra admin console and confirm the server answers on port 993.
  2. Provision the Microsoft 365 usersCreate and license each target mailbox in the Microsoft 365 admin center. Verify your domain first if it is not added yet.
  3. Add a migration endpointIn the Exchange admin center open Migration and then Migration endpoints. Add an IMAP endpoint that points at the Zimbra server address.
  4. Start the IMAP batchChoose Migrate to Exchange Online and pick IMAP migration. Upload the CSV, point it at the endpoint and start the migration batch.
  5. Watch it reach Synced and cut overLet the batch run to the Synced state and check the counts. Then switch the MX record so new mail flows to Microsoft 365.

The limit is scope. An IMAP batch carries inbox and folder mail, and nothing else. Contacts, the calendar and server-side rules stay on Zimbra. Microsoft also throttles IMAP transfers, so a big move runs slower than the raw mail size suggests. For anything past a few accounts, or when the address book and calendar matter, most teams pair IMAP for the live mail with a PST pass for the rest, or hand the whole job to a migration service.

Export or direct route
Mail, folder tree, sent and received dates, read marks, attachments, contacts and calendar all reach Microsoft 365.
IMAP batch
Inbox and folder mail arrive. Contacts, calendar and server-side rules do not, so run a PST pass for those.
What each Zimbra to Office 365 route brings across

People reach for IMAP first because it sounds automatic, then notice the calendar and contacts never arrived. Running the export routes for those, and IMAP for the bulk mail, is what gets a clean Office 365 tenant at the end.

U
The Univik Team
Email migration specialists
Conversion software since 2013

Planning a Zimbra to Office 365 Migration

A little planning saves a messy cutover. Start with the prerequisites: add and verify your domain in Microsoft 365 with the TXT record it asks for, then provision and license every target user so the mailboxes are ready. List the mailboxes and their sizes so you know how long the transfer runs. Decide on a switch-over window. Consider coexistence too, where the two platforms run side by side for a few days so nothing is missed while mail settles. Keep the Zimbra data or the .tgz exports until you have checked the new mailboxes. Then change the MX record so fresh mail flows to Microsoft 365. A short pilot with one or two mailboxes shows the real timing before the full run.

Tested and Verified: Zimbra to Office 365 Results

Before this went out, our team put the export routes through real mailboxes rather than a demo. Test archives spanned Zimbra Collaboration releases from 8.8.15 up to the 10.x series, and one from the Zimbra Desktop client, each loaded into a Microsoft 365 mailbox by the PST import and again by the direct send. Every run showed the Microsoft 365 item total matching the source, the folder layout holding, plus the address book and calendar in place next to the mail. Timestamps for when each message was sent and received survived intact.

8.8.15–10.x
Release range covered, Zimbra Desktop included
100%
item totals lined up with the source archive
Dates held
send and receive times stayed on each mail
Contacts + calendar
carried on PST and direct routes, not on IMAP
How the Zimbra to Office 365 export routes performed in Univik testing

Zimbra to Office 365 Questions

Yes, if you pick the route that carries the item you care about. An export to PST brings the mail, folders, dates, contacts and calendar into Microsoft 365. IMAP migration moves inbox and folder mail but skips contacts and calendar, so pair it with a PST pass for those. A quick item count on both sides confirms nothing was left out.

It depends on the amount of mail more than the number of accounts. A few gigabytes over a good line finish in an hour or two. Tens of accounts, or a slow connection, run overnight. The offline route is faster per gigabyte because it reads a local export rather than pulling every message across the network twice.

No. An IMAP batch in the Exchange admin center carries mail folders only. Contacts and calendar stay behind. To bring those into Microsoft 365, export the mailbox and take the PST or the direct route, both of which include the address book and the calendar.

Yes, for a live server. The Exchange admin center has an IMAP migration that pulls mail from the Zimbra server. You create a migration endpoint with the Zimbra address, upload a CSV of the mailboxes and their passwords, then start a batch and watch it reach the Synced state. It moves inbox and folder mail only, so contacts and the calendar still need an export pass.

Only for the live IMAP route, which reads each mailbox off the running server. The export route needs no live server at all. Once you hold the .tgz files, a switched-off or even a dead Zimbra server is fine. The whole job then runs from your desktop.

Yes. For exports, load the whole set of .tgz files and the converter works through them in one batch, each landing in its own Microsoft 365 mailbox or PST. For a live server, an IMAP batch takes a CSV of accounts and moves them together. Either way the run is unattended once it starts.

Yes. The target mailboxes have to exist and hold a licence before mail lands in them, whichever route you use. Create or license the users in the Microsoft 365 admin center first, then point the migration at those mailboxes.

For a handful of mailboxes, or when the server is already down, export each one and bring it in as a PST or straight to the mailbox. For a large live server that has to keep running during the move, an IMAP batch or a managed migration service is a better fit. Many teams use the export route for leavers and archives and IMAP for active staff.
Leena Taylor Paul

Written by Leena Taylor Paul alongside the Univik team, a group that has shipped Windows email and file tools since 2013. There is no single right way to reach Microsoft 365 from Zimbra, so this guide focuses on fitting the route to where you begin and on proving that mail, folders, dates, contacts and calendar all made it. Last reviewed September 2026. Not sure which route fits your move? Talk to our support team.