Can I easily and quickly import a large number of messages (say, 75k) from one IMAP-enabled email provider to Gmail?
Currently, all my archived messages live on Fastmail.fm, but I really miss some aspects of Google’s UX, including search. I’m on a Mac, but every time I try manually migrating (via Mail or Thunderbird) by copying the messages, I get timed-out or an error prevents them from properly migrating.
This isn’t just from Fastmail.fm to Gmail—I’ve encountered similar challenges when attempting to move a large number of messages to any IMAP provider. Does the IMAP protocol say anything specific about large numbers of messages within a given mailbox?