outgoing mail bounces with error 550 5.7.1

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?

Adding the From header in the mail body is a client function.
I would look at the configuration for NutsMail.
Presumably there is a setting for the default domain name.

but - surely there are mail servers that run without a webmail front end, running solely for pop3 or Imap connections. There must be some way of configuring postfix to output the address properly?

Postfix typically doesn't output the address at all (although it can be configured to rewrite addresses if required), it is the client (Tbird Outlook etc.) that constructs the mail body and also supplies the MAIL FROM: envelope sender address in the SMTP transaction (which can be different to the From address in the body).
The MTA (postfix) typically just passes these along as is.

There is a website that can test your DNS server settings, try:

www.solvedns.com

This may help to see if everything is set up correctly

Thanks for all of the replies. Turns out that this was heavily client related.. we also added an SPF record to our DNS, in order to make it all work neatly. I documented the solution in this thread:

http://hintsforums.macworld.com/show...205#post718205