From: Guy I work with 
To: Me
Bingo!
_____________________________________________
From: Me 
To: Guy I work with
Then they screwed up their reply-to, or they’ve configured that address to refuse incoming mail
since it’s just for mass mailings and they want you to contact someone in particular. Check
for “do not reply to this” in the original email, it may be there.
_____________________________________________
From: Guy I work with 
To: Me
Still doesn’t make sense, I replied to their email?
_____________________________________________
From: Me 
To: Guy I work with
It means the address you sent to is wrong according to [their mailserver]
_____________________________________________
From: Guy I work with 
To: Me
Kevin, what does this mean?
_____________________
From: System Administrator 
To: Guy I work with

Your message did not reach some or all of the intended recipients.
[blah blah error message]