Reliability
Stopping halfway looks worse than it is. No message is lost: the source mailbox was never touched, and the destination holds exactly what made it across.
An IMAP migration copies, it does not move. Messages on the source stay untouched from start to finish, whatever happens to the process.
Before copying anything, the tool checks what the destination already has and skips the matches. So a second run after an interruption does not start over: it walks the lists, sees what is already there, and copies the remainder.
That also makes the repeat cheap. If eight of ten gigabytes made it, the second pass costs the time of two, not of ten.
The causes are almost always on the provider or network side, not in the messages.
Five minutes of checking saves a second stop in the same place.
Do not delete what already arrived and start from an empty mailbox: that is precisely the work a repeat run knows how to skip.
Do not run several copies in parallel to catch up faster. The provider will refuse on connection count, and you get a third interruption instead of speed.
And do not delete the source mailbox before the counts match. While both sides exist, every mistake is reversible; after the source is gone, none of them are.
FAQ
No. The tool compares messages on the destination by identifier and headers and copies only the missing ones.
No. The migration only reads the source. Mirror mode, which can delete on the destination, is a separate opt-in and is never replayed automatically.
The pass still starts at the top of the list, but already-copied messages are skipped — in time terms, that is resuming.
That is a distinct error with its own write-up: either the daily export limit or space in the destination mailbox.
A separate password for IMAP instead of your account one: who requires it, where each provider keeps it, and what to do when the section will not appear.
Speed is capped by the two mail servers, not by the software. How to estimate the time in advance and what to expect from a mailbox of tens of gigabytes.
Labels versus folders and the All Mail folder: the number one cause of doubled volume and duplicates, and what to exclude before you start.
Connect two servers, look at the size and decide. Or take the client and keep everything on your side.
Code on GitHub · built on imapsync · no sign-up