I just found out that one of our servers, where a client runs a mailing-list script (opt-in list), is misconfigured.
Here's the thing:
His Perl scripts rely on sendmail,
They send messages containing a "From" field set to "email@example.com"
When the recipients read their messages, they see "From: firstname.lastname@example.org
"; well that's my understanding.
When an message is bounced (host unknown or mailbox full or ...) the bounce message is addressed to "clientdomain@OURdomain.com".
We rely on suexec, thus the Perl scripts as executed with the client's account, that could be resolved to "clientdomain@OURdomain.com"; I suppose sendmail uses this for the envelope "From" instead of the message "From" field..?