Give each mailbox what the other one is missing
One pass, both directions, no deletion anywhere. This tool only ever adds.
Read this first: deletions are not propagated
A single pass keeps no record of a previous one. So a message present on one side and absent on the other may just as well have been added here as deleted there — nothing lets the tool tell those apart. Rather than guess, it only ever adds: each mailbox receives what the other one has and it doesn't. Delete a message on one side and run the tool again, and it comes back from the other side.
- 1 Both mailboxes are inventoried. Headers only — never message contents.
- 2 Each message is matched by identity. Message-ID, or a header fingerprint when there is none — so a message filed differently on each side is not duplicated.
- 3 What's missing is copied, in both directions. Then it's over: nothing keeps running, and your credentials are erased.
How it works
- You enter two mailboxes — in whichever order you like: both are read, and both receive.
- Both are inventoried — headers only, never message contents. That is what makes it possible to handle a mailbox of tens of thousands of messages without downloading its gigabytes.
- Each message gets an identity — its Message-ID, or a fingerprint built from sender, recipient, subject, date and size when it has none.
- You see the estimated volume, then you start. Nothing moves before that click.
- What's missing is copied, in both directions — into a folder of the same name, created if needed, with the original date and the read, flagged and answered states.
It is a union, not a mirror. When it's over, each mailbox holds at least what it held before, plus what the other one had. Neither has lost anything.
Why deletions are not propagated
This is the central question of any two-way sync, and it deserves a straight answer rather than a setting buried in a menu.
Picture a message present in mailbox A and absent from mailbox B. Two stories explain that observation, exactly as well as each other: the message just arrived in A, or it was deleted from B. Nothing in the state of the two mailboxes tells you which one is true. That would require remembering what both mailboxes held during an earlier pass — and this tool is a one-off: it keeps no record, no identifier, no state between runs.
Faced with that ambiguity, a tool that propagated deletions would be wrong half the time, and wrong in the worst way: erasing messages the user had just received. A tool that only copies is also wrong half the time — but its mistake is to bring back a message you no longer wanted. You can delete it again in three seconds. The other mistake, you cannot undo.
Hence the rule, in one sentence: this tool adds, it never removes. And the consequence, said up front: if you delete a message on one side and then run the sync again, it comes back from the other side. For it to be really gone, delete it on both sides before running.
The same reasoning applies to flags. A message read on one side and unread on the other raises the question "which one wins?", undecidable for the same reasons. States therefore travel with new copies — a copy arrives in the state of its original — but messages already present on both sides are never touched.
Use cases
- Catch up after an interrupted migration — a transfer that stopped halfway leaves two partially identical mailboxes: one pass brings them level, without copying again what already arrived.
- Bring together two mailboxes used in parallel — when two addresses have each lived their own life for a few months and you want both to have everything.
- Check that a copy is complete — run the sync: if it copies nothing, the two mailboxes already hold the same messages.
- Top up a standby mailbox — a mirror mailbox you refresh from time to time, with no automation and no permanent monitoring.
What the tool does not do / limits
- This is not a continuous sync. It is a single pass, which you start and which finishes. Nothing keeps running afterwards, and no credentials are kept.
- Deletions are not propagated, for the reason set out above. It is not an option to switch on: it is a property of the tool.
- Filing is not aligned. A message kept in "Clients" on one side and "Quotes" on the other stays where it is on both sides — it is simply not duplicated. Deciding which filing wins would raise the same undecidable question.
- Trash, spam and drafts are excluded on both sides, along with the "[Gmail]" space. Copying a trash folder across would make deliberately discarded messages reappear in the other mailbox.
- The tool only works over IMAP.
- Free up to 5 GB copied, both directions sharing a single allowance. Beyond that, the operation stops cleanly and can be finished for €5.99 incl. VAT, or with no limit on Premium. This threshold is the same for everyone.
Frequently asked questions
Does this sync delete anything? +
Never, on either side. The tool can do exactly one thing: add to a mailbox what it doesn't have and the other one does. No deletion is possible, not even by accident: the code has no delete command.
Why aren't deletions propagated? +
Because that would be guesswork. This sync is a one-off: it keeps no record of a previous pass. A message present on the left and absent on the right may just as well have been added on the left as deleted on the right — nothing the tool can observe tells them apart. Propagating deletions would be a coin toss, wrong half the time and irreversibly so. So we copy, and we don't delete: it is the only behaviour we can guarantee to be correct.
What happens if I delete a message and run the tool again? +
It comes back, from the other mailbox. That follows directly from the previous answer, and it is better to know beforehand. If you want a message gone from both sides, delete it on both sides before running the sync.
Is this a continuous sync? +
No. It is a single pass, started by you, that finishes and stops. Nothing keeps running afterwards, no credentials are kept, no monitoring is set up. To line the two mailboxes up again later, run the tool again.
What about read, flagged and answered states? +
They travel with new copies: a copied message arrives in the state it was in on its original side, with its original date. Messages already present on both sides are not touched — deciding whether one side's "read" or the other side's "unread" wins raises exactly the same problem as deletions, and gets the same answer: we don't guess.
Is a message filed in a different folder on each side duplicated? +
No. Matching is done on message identity (its Message-ID, or a header fingerprint if it has none), not on location. A message present on the other side, wherever it is filed, is not copied again. The tool does not move messages around to line filing up either: it leaves them alone.
Which folders are involved? +
All of them, except trash, spam, drafts and the "[Gmail]" space ("All Mail", "Starred", "Important"), which are views rather than folders. Copying a trash folder across would make deliberately discarded messages reappear in the other mailbox. The report lists the folders read and those left out, on both sides.
What does it cost? +
Free up to 5 GB copied, both directions combined — they share a single allowance. Beyond that, the operation stops cleanly and can be finished for €5.99 incl. VAT, or with no limit on Premium. The threshold is the same for everyone.