squirrelmail login failing

[t-mug]

I've got a fresh ISPconfig installation here; the squirrelmail package from http://ispconfig.org/downloads.htm is installed additionally.
But my web login via the https://domain.tld:81/squirrelmail is always failing: "Unknown user or password incorrect."
The /var/log/mail.log says:

Jun 13 13:27:02 ... imapd: Connection, ip=[::ffff:127.0.0.1]
Jun 13 13:27:02 ... imapd: LOGIN FAILED, user=web1_test@mydomain.tld, ip=[::ffff:127.0.0.1]
Jun 13 13:27:07 ... imapd: LOGOUT, ip=[::ffff:127.0.0.1], rcvd=63, sent=332

The login would probably work if the "user" value would be "web1_test" only (without "@mydomain.tld"), so like it does via my web client. The point is: I've put in the login username box of squirrelmail only "web1_test" as username - it seems that squirrelmail adds automatically this "@mydomain.tld" stuff. Are there any suggestions, how to solve this?

Thanks a lot