Univik MBOX Splitter for Windows
At a Glance
Key Features
Splits a Takeout MBOX by Gmail Label
Univik MBOX Splitter writes one file per label, nested labels as subfolders such as Clients\Acme.mbox. A message with several labels goes into every label file as in Gmail, or once with no duplicate messages.
At a Glance
Univik MBOX Splitter at a Glance Seven split modes, zero uploads, 10 MB of memory, version 1.0, $39
Seven ways to split, zero uploads, every file checked.
7
Split Modes
Size, count, label, year, month, sender, domain.
0
Files Uploaded
Every split runs on your PC.
10 MB
Memory Used
On a 1.1 GB export of 913 messages.
v1.0
Windows 11 and 10
64-bit, .NET Framework 4.8, every part read back.
$39
One-Time Licence
Free to try, one payment, 30-day money back.
A 1.1 GB Takeout export, listed by label
Key Features
What Does Univik MBOX Splitter Do Six features, from a Gmail label split to a read-back check
Keeps Conversations Together
A message and its replies always land in the same file. Threads come from Gmail's thread id (the X-GM-THRID header) or from the References and In-Reply-To headers.
Reads Every File Back and Checks It
After writing, Univik MBOX Splitter opens each part again and compares it with the source: same message count, same content. The report accounts for every message and split-index.csv lists where each one went.
Never Changes the Source File
The original is only read. Parts go into an Output folder, one subfolder per split, so a second run never overwrites the first.
Opens the Takeout Zip, No Unzipping
Univik MBOX Splitter reads the mbox inside the takeout-...-001.zip and -002.zip parts as it is, with nothing to unpack first.
Browse, Search and Leave Out
The tool lists the file by label, year and sender and previews any message. Skip Spam, Trash, chats and duplicates or keep a date range.
The Short Version
The Short VersionWhy Split a Large MBOX File Thunderbird skips a file past 4 GB and Apple Mail cuts off the end
Google Takeout gives you all of Gmail as one file, often too large to import. Big files are also slow to open and search.
Problem 1: Thunderbird Skips It
The ImportExportTools NG add-on reports Mbox larger than 4GB, skipping and the folder stays empty. The known workaround, copying the file into the profile folder by hand, still leaves one huge folder. See the add-on's bug reports.
Problem 2: Apple Mail Cuts Off the End
Apple Mail stops a mailbox export and import near 2.15 GB and every message after that point is lost. Documented in Apple Support Community threads.
Univik MBOX Splitter cuts the file into smaller parts under 2 GB or 4 GB, or into one file per label or year. Every part then imports. A cut is only made between two messages, never in the middle of one.
What It Reads
Which MBOX Files Can Be Split Google Takeout, Thunderbird profile files and Apple Mail exports
Google Takeout
All mail Including Spam and Trash.mbox
Zip parts open directly from the download. Labels come from X-Gmail-Labels, conversations from Gmail's thread id. Any standard mbox opens too, see the MBOX file format page.
Thunderbird
Profile folder files, no extension
Choose the profile folder and every mail folder inside opens.
Apple Mail
Exported .mbox mailboxes
Open the export folder and the mbox inside is read. Split it into parts under 2 GB before importing it back.
Open a file, the Takeout zip or a folder
Three Steps
Three StepsHow to Split a Takeout MBOX Three steps in Univik MBOX Splitter: open, choose, split
Three clicks on the PC, no sign-in and no mail program needed.
1
Open the MBOX, Zip or Folder
Click Open and pick an .mbox file, the Takeout zip parts or a folder of MBOX files. The software lists it by label, year and sender in seconds.
2
Choose How to Split
Pick size, count, Gmail label, year, month, sender or domain. Keep conversations together and leave out spam, trash or duplicates. The preview lists every file it will make.
3
Pick a Folder and Split
Choose a destination. The software writes an Output folder, reads every file back and saves split-index.csv.
Step 1: pick a split mode
Step 2: label options and preview
Step 3: destination, read-back check, index
Real Situations
When You Need an MBOX Splitter Tool A Thunderbird move, a file for your lawyer or a yearly archive
Moving Gmail to Thunderbird
Split a ten-year Gmail export by label so Inbox and Sent arrive as folders under the 4 GB import limit.
Sending Mail to a Lawyer or Accountant
Split by sender domain and every message from one client sits in its own file. Send that file only.
Archiving Year by Year
One file per year keeps an archive small enough for a FAT32 USB stick, and the index says which year holds what.
Read This First
Why the Takeout File Is So Big Labels are stored inside each message, so nothing is sorted for you
One file for everything
Google Takeout puts every Gmail message, Spam and Trash included, into one file named All mail Including Spam and Trash.mbox, often many gigabytes. Takeout then cuts the download into zip parts, 2 GB each by default, named takeout-...-001.zip and -002.zip.
This MBOX file splitter reads the zipped or unzipped file a little at a time. It writes the parts you choose into Output\All mail Including Spam and Trash - split by Gmail label\ with split-index.csv beside them.
Compare Your Options
Compare Your OptionsThree Ways to Split an MBOX File Free Python scripts, a mail client re-export or Univik MBOX Splitter
| What you need | Python mailbox Module Script | Thunderbird + ImportExportTools NG | Univik MBOX SplitterTRIAL |
|---|---|---|---|
| Split by Gmail label | Only if you code it | Manual, folder by folder | Yes, like Gmail |
| Opens the Takeout zip parts | No, unzip first | No, unzip and import first | Yes |
| File over 4 GB | Yes | Skipped by the add-on | Yes, read at 10 MB of memory |
| Keep conversations together | No | No | Yes |
| Every file read back and checked | No | No | Yes |
| Index of where each message went | No | No | split-index.csv |
| Install needed | Python 3 | Thunderbird and the add-on | One installer, no mail program |
| Mac or Linux | Yes | Yes | No, Windows only |
| Cost | Free | Free | Free trial, $39 licence |
Tested
TestedTested on a Real Takeout Export Univik MBOX Splitter on a 1.1 GB Gmail export, 913 messages, 7 labels
A real 1.1 GB Google Takeout export opened in Univik MBOX Splitter: 913 messages, 7 labels, 615 conversations, 2017 to 2020. It was read in one second with 10 MB in memory.
| Split mode on this export | Files it makes | Note |
|---|---|---|
| By Gmail label | 13 | 1,162 copies of 913 messages with a copy in every label file, 1.3 GB in all |
| By year | 4 | 2017 to 2020, 302 messages in the largest year |
| By sender domain | 24 | gmail.com alone held 476 messages |
| By sender | 62 | Small senders grouped into Other senders |
Test setup: Univik MBOX Splitter 1.0, 64-bit Windows 11, September 2026, our own Takeout export. Counts come from the split preview and will differ for your file.
The split report: every message accounted for
What You Need
MBOX Splitter System Requirements 64-bit Windows 11 or 10, .NET Framework 4.8, no mail client
Windows Version
Runs on 64-bit Windows 11 or 10 and needs .NET Framework 4.8 or higher.
No Mail Client Needed
This MBOX splitter tool reads the file on its own. No Thunderbird, Outlook or Apple Mail needed.
Memory and Disk
The file is read a little at a time, so 4 GB of RAM is enough. The Save step shows the disk space needed first.
Runs Offline
The internet is needed only to download the installer.
| Software Information | Univik MBOX Splitter |
| Version | 1.0 (Latest) |
| Licence | $39 one-time, free trial first |
| Platform | 64-bit Windows 11 or 10 |
| Framework | .NET Framework 4.8 or higher |
| Reads | MBOX files, Takeout zip parts and whole folders of mbox files |
| Writes | Standard .mbox parts plus split-index.csv and a split report |
| Split Modes | Size, message count, Gmail label, year, month, sender, sender domain |
Questions
QuestionsMBOX Splitter Questions Takeout, labels, the 4 GB limit, split-index.csv and the trial
Open the Takeout file in Univik MBOX Splitter, choose Gmail label, year or size, pick a folder and click Split. The parts are saved in an Output folder, each read back and checked, with the original untouched.
Yes. The tool reads the X-Gmail-Labels header and makes one file per label, nested labels as subfolders. A message with several labels goes into every label file, or once with no duplicates.
No. The software only reads the source and never writes to it. New files go into an Output folder, one subfolder per split, so a second split never overwrites the first.
The ImportExportTools NG add-on reports an mbox larger than 4 GB as skipped and leaves the folder empty. Split it into parts under 4 GB or by label or year and each part imports.
Apple Mail stops a mailbox export and import near 2.15 GB, and every message after that point is lost. The size split has a 2 GB preset for this, so every part stays under the limit.
A message and its replies always land in the same file. The tool groups by Gmail's thread id or by the References and In-Reply-To headers, so no conversation is split across two files.
The trial runs the full split and writes the first 10 messages of every output file. Names and folders are the same as in the paid version. A one-time $39 licence writes every message.
No. Pick the takeout zip parts, such as -001.zip and -002.zip, and the splitter reads the mbox inside them as they are. A folder of files opens the same way.
No. Univik MBOX Splitter is Windows software for 64-bit Windows 11 or 10. On a Mac, split the file on any Windows PC and copy the parts over. The parts are standard mbox, so Apple Mail and Thunderbird import them.
No. Univik MBOX Splitter reads the mbox file on its own. No mail program, no Python and no internet connection are needed. The only requirement is .NET Framework 4.8, which Windows 11 and an updated Windows 10 already have.
Yes. The tool works offline, the source file is only read and nothing leaves the PC. The installer carries a code signature and installs nothing extra. A 30-day money-back guarantee covers the licence.
Guides
GuidesTakeout Splitting Guides Nine guides on Takeout, Thunderbird and Apple Mail problems
- How to Split a Large MBOX File
- Thunderbird Says the File Is Larger Than 4 GB
- The Apple Mail 2 GB Mailbox Limit
- Split a Takeout Export by Gmail Label
- Split Mail by Year or Month
- Split Mail by Sender or Domain
- Google Takeout Export Too Large to Open
- Free Splitter Scripts vs a Desktop Tool
- Import a Takeout Export into Thunderbird
More Tools
More ToolsMore Univik Mail Tools MBOX Converter, MBOX Viewer and a free online viewer
Univik MBOX Splitter in Short
- Splits one large MBOX file, such as a Google Takeout export, by Gmail label, year, month, sender, domain, size or count.
- Opens Takeout zip parts and whole folders directly, offline on Windows.
- Reads every part back against the source and writes split-index.csv.
- Free trial writes 10 messages per file. Full licence $39, one payment.
About This Page and Its Sources
Fact Checked
Written and checked by the Univik team, makers of file tools since 2013. Import limits come from the ImportExportTools NG issue tracker and Apple Support Community threads. The test figures are from our own export.
“We built the MBOX Splitter around one rule: never trust a split you cannot check. Every file is read back and compared with the source, and the index tells you where each message went. If a tool cannot prove that, it should not be cutting your mail.”
Our installer carries a code signature and installs nothing extra. A 30-day money-back guarantee covers every licence.
Since 2013Read-Back VerifiedUnivik Team
Updated September 2026