I am having trouble with email. My network (NT4) is running Exchange 5.5

I send email out and everything works no problem. My problem is that some emails dont go. I have a couple of addresses that do not work. I send the message and it comes back undeliverable.

The text of the NDR is"Your message did not reach some or all of the intended recipients.

Subject:
Sent: 30/01/01 13:46

The following recipient(s) could not be reached:

'[email protected]' on 30/01/01 13:46
Unable to deliver the message due to a communications failure
The MTS-ID of the original message is: c=US;a= ;p=Shachter-Namdar;l=NT_EXCHANGE1-010130114549Z-1481"

The email address is correct. I replaced the user name with xxxx, so as to protect his anonymity.
BTW, if I email him from my home email address, it works fine and gets delivered.

What would cause this? This is not the only recipient I have trouble with. There is another one, and the message is the same.

Any ideas?


Thanks
Rafi