I decided to switch back to Gmail/Inbox by Gmail. Copying thousands of mails in Thunderbird is a pain and after a couple of unsuccessful tries I started to search a better working alternative. mutt to the rescue!
# login to your old imap account: mutt -f imaps://username@oldmailserver/INBOX/folder # mark all messages in that folder (otherwise use "t" to select single messages) enter "T" and then "~A" # either copy (;C) or move (;s) the messages. ;s will mark them as deleted on the old account! enter ";C" # as a target directory choose your gmail account: imaps://firstname.lastname@example.org/[Gmail]/folder
You may want to start mutt in a "screen" session and then let it run for a while. On linux systems mutt should already be installed. On OSX you can find it in homebrew.