thanks! We are getting somewhere.
Webmail (nutsmail) is writing an additional from" header. When I remove this setting in NutsMail, I'm left with 1x From: header. Great!
However, as soon as I remove it from nutsmail, all outgoing email is marked with @mail.mydomain.com instead of @mydomain.com.
The above postconf -n post looks correct to me, so where/how do I change this?