How to Split an MBOX File by Sender or Domain (2026)

Quick Answer

A sender split reads the From address on every message and drops each one into a file named for that sender. A domain mode instead gathers everyone at the same company into a single file, rather than making one file per person.

This is the fast way to pull every message from one client or vendor into a file of its own, without dragging the rest of the mailbox along with it.

Sometimes you need only the mail from one person or one company, not the whole mailbox. A lawyer asks for everything you exchanged with a supplier. An accountant wants the invoices from one vendor. A sender split reads the From address on each message and sorts the mail into a file per sender, so the people you care about come out cleanly. This guide covers the difference between a sender split and a domain split, how to stop a flood of tiny files and how to build the result with the Univik MBOX Splitter.

A mail archive grouped into separate sender files on one side and a single company-domain archive on the other. — Sender mode keeps one file per address; domain mode gathers people from the same company into one archive.

What Is the Difference Between a Sender and a Domain Split?

A sender split makes one file for each From address. A domain split makes one file for each company. If three people at acme.com wrote to you, a sender split gives you three files, one per person, while a domain split gathers all three into a single acme.com file. Pick sender when you want each contact kept apart. Pick domain when you think of the mail by the company it came from rather than the individual. On one test mailbox the sender mode produced sixty-two files and the domain mode produced twenty-four, because several people shared each company address.

The By senders view in Univik MBOX Splitter — **In the app:** the Senders view groups mail by address and by domain with a count each, so you can see who wrote the most before a sender split.

What you want	Sender split	Domain split
Groups mail by	Each From address	Each company address
Files from one company	One per person	One for the whole company
Number of files	More	Fewer
Best for	Keeping each contact apart	All mail from one firm

How Do You Avoid Thousands of Tiny Sender Files?

A large mailbox holds mail from hundreds of one-off addresses, and a plain sender split would make a separate file for every one of them. That is a mess nobody wants. The fix is a floor. You set a number such as five. Any sender who wrote fewer than that many messages goes into a single Other senders file rather than getting a file of their own. The people who matter, the ones who wrote to you often, still get their own files. The long tail of newsletters and receipts collapses into one. You keep the files that are worth keeping and skip the clutter.

How Do You Hand One Sender's Mail to a Lawyer or Accountant?

When someone asks for all your mail with one company, a domain split gives you exactly the file to send. You run the split on the whole mailbox, find the file named for that company and send only that file. The rest of your mail stays private on your own machine. Before you send it, open the file in a viewer and check that it holds what you expect. Count the messages so you can say how many there are. Sending one focused file is cleaner and safer than forwarding a pile of messages or handing over the entire mailbox and trusting the other side to look only at the part that concerns them.

How Do You Split an MBOX by Sender on Windows?

On a Windows PC the Univik MBOX Splitter does a sender or domain split in a few clicks. It looks at the From address on every message, gathers the mail the way you pick and saves a file for each sender or company inside a new folder. Each written file is opened again and matched to the source, which shows that none of the mail went astray. A split-index.csv file records which sender each message belongs to. The steps below walk through it.

Open your mailbox

Point the tool at the .mbox file. It shows each sender and how many messages they sent, so you can see who wrote the most first.

Choose sender or domain

Pick a split by sender for one file per person, or by domain for one file per company. Set a floor so rare senders land in an Other file.

Choose a destination and start

Set the folder for the output. The tool saves one file per sender or company, matches each one to the source and records every message inside split-index.csv.

One file per sender or company

Univik MBOX Splitter splits by sender or sender domain and pools the small senders into one Other file.

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Which Script Splits an MBOX by Sender?

Yes, a short script can do a sender split. Python ships with a module called mailbox that reads an MBOX file message by message. A few lines can look at the From header on each message and write it to a file named after that sender. Anyone comfortable writing Python can build this in an afternoon. The catch is what the script leaves out. It does not check that every message reached a file, it does not pool the one-off senders into an Other file unless you code that too, and a mistake in how it copies each message can damage the output. A desktop tool handles the pooling, the check and the index for you, which is the trade for the licence fee.

Key Takeaways

A sender split makes one file per From address; a domain split makes one file per company.
On a test mailbox the sender mode made sixty-two files and the domain mode made twenty-four.
Set a floor so rare senders collapse into one Other file instead of thousands of tiny ones.
A domain split gives you one file to hand a lawyer or accountant, without sending the rest.
Check and count a file before you send it, so you know what is inside.
Python's mailbox module can script a sender split, but it will not pool, check or index for you.

When a lawyer asks for everything from one company, send one file that holds only that company. Not the whole mailbox with a promise.

Nick Rogers

Founder, Univik

Email handover tools since 2013

MBOX Sender Split Questions

Run a sender split. It reads the From address on every message and saves one file for each sender. Find the file named for that person and you have every message they sent, kept apart from the rest of the mailbox.

A sender split makes a file for each From address, so three people at one company give you three files. A domain split gathers everyone at that company into a single file. Choose domain when you think of the mail by firm, sender when you want each contact separate.

Set a floor. Any sender who wrote fewer than the number you pick goes into one Other senders file rather than a file of their own. Frequent contacts keep their own files while the long tail collapses into one.

Run a domain split on your mailbox, take the file named for that company and send only that file. The rest of your mail stays on your own machine. Open the file and count the messages first so you know what it holds.

Yes. Python's mailbox module reads a mailbox message by message, and a short script can write each one to a file named after its From address. It will not pool rare senders, check the output or index the messages unless you add that code.

Yes. Each file is a standard mbox, so it opens in any mail program that reads mbox. A tool that reads every part back after writing confirms the messages came through whole, with the same content as the source.

No. The split only reads the source and writes the new files into a separate folder. The Univik MBOX Splitter never writes back over the file you started with.

Leena Taylor Paul

The Univik crew put this sender-split guide together with Leena Taylor Paul, and they have built email handover and export software for Windows since 2013. The sender and domain counts come from our own test mailbox. Last checked September 2026. Need one sender pulled out of a huge file? Contact our support team.