Use alias domain for (pop3) login: How?

[insel22]

Hello.

It is possible to use an alias domain for (pop3) login? (I hope, it is.)

I have a domain example.com and a domain alias example.net. Mail to foo@example.net is accepted and delivered to the mailbox of foo@example.com. No problem here.

But: I want to be able to use the address foo@example.net (with example.net = alias domain) for pop3 authentication. How do I do that?

Login with main domain is working:

# telnet localhost 110
[...]
user foo@example.com
+OK Password required.
pass verysecret
+OK logged in.

Login with alias domain fails:

# telnet localhost 110
[...]
user foo@example.net
+OK Password required.
pass verysecret
-ERR Login failed.

Hints are welcome.
Thanks in advance.