Is there a way I can make joomla do the following:
I have several mail accounts that should act as a discussion mailing-list. The administrator (and only the adminstrator) maintains a list of customer email adresses that belong a particular discussion group mail adress. Whenever a Mail is sent to e.g. specialinterestdiscussion(at)mydomain.com, the mail will automatically be forwarded to all customers that belong to that list. If the sender is part of that mailling-list it will not be forwarded to him again. The sender does not necessarily have to be part of the maintained list.
So basically it is a simle mailing list.
Since Joomla is a php based engine it is simply not not possible. Joomla itself can not run a service in the background which would be required for a mailing list.