Markus Gaugusch schreef:
In your case, you could use a transport table entry in your mail server and send all gmx mails to gmx and still use your server as central mail sending hub.
That won't work in many cases, since transport tables usually work based on the recipient address and not the sender address (which is what you need in case your MTA has multiple upstream relays). Postfix apparently has a configuration parameter (I forgot the name and have no time now to search for it) to reverse the meaning of the transport table, but the author recommends strongly against using it. The best solution to this problem is to configure the upstream accounts in a MUA and not a MTA. If you really feel the need to run an MTA with multiple upstream relays, a tool called 'esmtp' (http://esmtp.sourceforge.net/) may be of help. It will allow you to configure multiple upstream hosts, depending on the sender address instead of the recipient address as in the Postfix 'transport' table. Configuration examples for commonly used MTA's are included in the package.