While I see lots and lots of folk complaining about this sort of thing not working. I rcan not offer them any really helpful advice other than to point out that imap is a protocol designed to synchronize email, not archives of email. Practical experience indicates some 50 to 100 email will synchronize this way without issue on most occasions, Dropping larger batches will work for some, up to around 500. Beyond that emails are dropped and lost all the time because the protocol is being used for something it is not designed for. Basically you need Google to come up with an import for their own email archives. They have twp projects on github that do the grunt work, but they do not support them. https://github.com/google/mail-importer and https://github.com/google/import-mailbox-to-gmail The latter is for importing their own exported mbox archives for when you change accounts. Just what you are trying to do I think.