smtp.relay.tld mailbox@domain2.tld:mailpass

+----+----------+-------------+----------+
| id | username |   domain    | password |
+----+----------+-------------+----------+
|  1 | mailbox  | domain2.tld | mailpass |
+----+----------+-------------+----------+

user = sqluser
password = dbpass
hosts = localhost
dbname = maildb
query = SELECT password FROM accounts WHERE username = '%u' AND domain = '%d'

query = SELECT CONCAT(username, ":", password) FROM accounts WHERE username = '%u' AND domain = '%d'