SMTP no-such-user issues

[steve at ibctech.ca (Steve Bertrand)]

Steve Bertrand wrote:
> Hi everyone,
> 
> We are experiencing an issue in regards to SMTP MTA relay responses 
> regarding 'no such user', and it *apparently* appears to be only 
> occurring when a particular site attempts to deliver email to us. Any 
> advice on how to further troubleshoot my issue would be greatly 
> appreciated.

Because I have been inundated with off-list replies, I'll post my 
findings here.

First, a screen cap of me composing a message, in which two of the 
addresses are easily identified as valid, and two are clearly not:

http://ibctech.ca/nanog_smtp/hotmail_compose.jpg

...when I sent it, I received thus:

http://ibctech.ca/nanog_smtp/hotmail_response.txt

So, we'll try it from a different server:

http://ibctech.ca/nanog_smtp/local_compose.jpg

...hmmm, I received the message at steveb at eagle.ca (even though the To: 
field was cropped out of the jpg) this time, and my server generated the 
following....AFAICT, correct 551 error(s):

http://ibctech.ca/nanog_smtp/local_err.txt

I'm willing to provide any more documentation required in order to get 
this issue sorted.

Thank you everyone, I truly appreciate it.

Steve