RT:Autoreply doesn't work

we just installed RT to test it for our purposes handling customers requests as a little software company.
Now we got the problem, that the autoreply feature doesn’t work for automatic incoming requests where the new tickets
are created automatically.
The error message is
Jul 15 10:11:43 isvrouter RT: rt-3.4.2-229-1893-3.15.9103103760311@isv-gmbh.de No recipients found. Not sending.

First we tried it from a company e-mail account.
And then from a GMX account.
In both cases we got the same error message.

Any ideas what can be wrong?