I am using the following recipe in my .procmailrc file to forward a copy of all email messages to another account:
:0 c ! somebodyelse
This works fine. But if I try to use forward to an external email address:
:0 c ! email@example.com
It does not work. Procmail seems to send the mail fine, postfix (on the same computer) then forwards it to an smtp smart host which apparently accepts it without errors, but it never arrives.