If you're upgrading to Mac OS X Server 10.3 and select the mail database checkbox in AppleShare IP Migration, the migration won't finish. Some Apple documentation mistakenly says this feature works, but in reality it doesn't for a migration to 10.3. Here's a couple of simple workarounds:
First, you could temporarily have both AppleShare IP and Mac OS X Server mail servers running, ask clients to configure both email accounts for IMAP, and then simply copy messages between the two.
Second, you could migrate the AppleShare IP mail database to Mac OS X Server 10.2.8 or earlier and then migrate the new database to Mac OS X Server 10.3.x.