[Dovecot] DIGEST-MD5 user/realm mismatch with Postfix

This is a follow-on to the "SQL field format for digest-md5?" thread.

After some additional debugging, I've found the problem is a mismatch in what Dovecot expects and Postfix uses for the user and realm.

For a username "user@example.com" and password "sekret", Dovecot expects:

username = "user"
realm = "example.com"

but Postfix sends the checksum of:

username = "user@example.com"
realm = ""

How do I fix this?