[exim] Problem sending Mail to comcast

Hello List,

i have much trouble sending to comcast Mailservers. The comcast Servers
returns "450 [TEMPFAIL] comcast.net requires valid sender domain".
Sometimes the mail goes thru after a few hours but mostly not. What i
can see with ngrep is that my exim must wait a long time for Return
Messages and then send line to the comcast servers like this

"From:<xyz@???> To:<xyz@???> ..."
and does not wait for return code, so comacst servers saw the To-Header
not correctly. I think that the problem is on comcast Side but how to
explain my customers.

Anybody an idea?

Regards

Tom