postfix sasl login with several domains

I've set up a mail server with yosemite's server app which hosts my two domains, say myfirstdomain.com and myseconddomain.com where myfirstdomain.com is the canonical domain name of my server.

Assumed i have a user "peter" i can now login to IMAP with peter@myfirstdomain.com and peter@myseconddomain.com but only if i use "plain" as authentication method. It doesn't work if i use for example "MD5 Challenge Response" where i can only login with peter@myfirstdomain.com. And SMTP login doesn't accept peter@myseconddomain.com as user name at all, only peter@myfirstdomain.com.

Can somebody explain me this behaviour? And how can i achieve that i can login with both domains? (Sorry for the bad english)