Your going to have to break it down a little more by telneting into your mailserver from the machine your having trouble with, and sending a hand type email from the command prompt. You should see the error appear, and be able to read the message.
Don't use the backspace key, and get it right the first time.
telnet SMTP.domain.xyz 25
helo SMTP.domain.xyz
mail from: f@domain.com
rcpt to: t@domain.com
data
subject: -type-
.
quit
You can use the smtp exception class, to get more precise information on what's happening, I had to do these things last week to figure out why I could not send email to certain domains.
[
smtp exception class^]