Originally Posted by erk
Or do you have to set external mailserver for both the main domain AND all co-domains for the setting to work?
Yes. The logic behind that is very simple.
The website has the domain www.example.com
. If you set mailserver to external in the website, the local server does not accept mails for ...@www.example.com anymore. If you have a co-domain example.com (with empty host), you will have to set this to extaernal mailserver if you dont want to get emails for this domain (example.com) on the local mailserver.