After fooling around with it for a couple of hours I've figured it out.
I edited /etc/authlib/authmysqlrc
MYSQL_SELECT_CLAUSE SELECT email,password,"",uid,gid,homedir,maildir,quota,"" ,concat('disableimap=',disableimap,',disablepop3=' ,disablepop3) FROM mail_user WHERE email = '$(local_part)@$(domain)' or email2 = '$(local_part)'
Then added email2 column to the mail_user table in the dbispconfig database and filled the rows with the user+domain.com usernames.
tested with multiple accounts using both types of usernames in outlook.