In the UE2 server behavior, if you go into the "Mail Object Settings", there is a check mark for the Include from address using additional parameters. I assumed this would be if the emails are not going through and required the senders email address and password?
I have used the same domain for the "from" and the "to", but they do not get received. If I change the "to" to another email address on an external domain, they arrive.
I read another posting here from Jason that made reference to some email services not liking if both sender and recipient are on the same domain. That made sense to me because the client is not receiving the form on his own domain, but I am receiving it if I use my email address which is separate from theirs.