[Marcus C. Gottwald]

I was sure that none of the involved MTAs that I have control over did something like that. But when I was digging into it, I found that all mails were addressed to listname@main-fqdn although had been sent to listname@some-other-fqdn. This happens when the other fqdn is not a "real" DNS-entry, but a CNAME.

Using CNAMEs in mail addresses is Wrong. So don't. Cf RFC 1123, section 5.2.2:

  5.2.2  Canonicalization: RFC-821 Section 3.1

     The domain names that a Sender-SMTP sends in MAIL and RCPT
     commands MUST have been  "canonicalized," i.e., they must be
     fully-qualified principal names or domain literals, not
     nicknames or domain abbreviations.  A canonicalized name either
     identifies a host directly or is an MX name; it cannot be a
     CNAME.

-- Harald