we did our migration from plesk to smartermail. unfortunately, we had 2 different versions of plesks, 7.something and 11.something.
the one on 7.something didn't offer any form of migration tool, as such, we had to re-create accounts by exporting the email accounts to excel and then importing them to smartermail.
as for the 11.something, we did a migration and it was pretty smooth. in some cases where we couldn't do a direct transfer, we used imap migration to map the mailboxes from old mail server to the new set-up in smartermail.