Move mail in from another provider

There is no import tool yet. What works today is your mail client: add both accounts, then drag or copy folders from the old one to the new. The client does the transfer over IMAP, and it moves the messages with their dates and read state intact.

Thunderbird handles large mailboxes best of the free options — it copies folder by folder and survives being interrupted. For very large accounts, move one folder at a time rather than the whole tree at once.

This works because both ends speak IMAP, which is also why leaving later works the same way. There is nothing to request and nobody to ask.