Scenario: The old IMAP Server is no longer reachable. There's one Mac that has had access to all company accounts and holds copies of all emails, fully sync until the minute the old mail servers have been shut down. There are new mailservers now (entirely different data center). All mail accounts and addresses have been created on the new server. Unfortunately, since the old data center (1&1) has dropped all associated data (the domain has been moved as well), we'd like to copy the local copies of the old, offline accounts (mails still showing and opening with all attachments) into the new IMAP accounts which have as well all been added to said Mac. This all sounded too easy to work from scratch, Apple Mail hangs in "preparing to copy" from the offline accounts. Any ideas? PS feel free to improve my question, my english is not the very best and I am yet not too familiar with formatting text on stack either. Since I believe I won't stay the only one with this problem and since I did not find a similar question that fixes my issue, I think this is the correct place to ask for advice. Cheers in advance.
Edit: It must be possible somehow, because a few mails have been copied after ages of waiting. There must be a way to do this faster. I've copied like 10 out of 1000+ E-Mails yet.