Postfix + sasl method rimap strips domain name from user
You probably just need to add -r to your saslauthd OPTIONS= stanza.
From the man pages
-r Combine the realm with the login (with an ’@’ sign in between).
e.g. login: "foo" realm: "bar" will get passed as login:
"foo@bar". Note that the realm will still be passed, which may
lead to unexpected behavior.