Email aborting

I've been running into a problem lately...when I try to send an email from one domain on my account to another, I get a Delivery Notification - Delivery to the following recipients was aborted after 19 second(s): etc...
The odd thing is, I think it has to do with domain links (ie. in people's signatures). when I strip those out, the emails seem to go through. I can't figure out why?! Or how to fix it.
The odd thing is, I think it has to do with domain links (ie. in people's signatures). when I strip those out, the emails seem to go through. I can't figure out why?! Or how to fix it.