Verification
A migration that finished without errors is not yet proof that everything is there. The check takes ten minutes, happens once, and lets you close the old mailbox without second thoughts.
The first thing people do after a migration is compare gigabytes. They almost always find a difference and draw the wrong conclusion from it.
Providers account for used space differently: some include indexes, some round, some deduplicate attachments. Add the headers the receiving server writes itself.
So the unit of verification is the message, not the byte. Message counts per folder should match; sizes are not required to.
Done once after the final pass, a few minutes of work.
Four discrepancies come up most often, each with a clear cause.
Not immediately. Sensible practice is to leave the source untouched for two to four weeks after the MX switch. That is when the rare messages nobody thought about on moving day surface.
While both sides exist, any mistake is fixable with another pass. Once the source is gone there is nothing to fix from — and that is the step where mail is genuinely lost.
FAQ
Ideally none. Differences usually trace back to a message-size limit, or to Spam and Trash that were excluded on purpose.
Usually, but not with every provider. That is a property of the receiving side, not of the migration.
The receiving server adds its own headers and may count attachments differently. Compare messages, not bytes.
Yes, there is a counting mode that reads folder and message lists and writes nothing.
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