Without seeing the entire header, esp. the sender info, it would be guesswork to troubleshoot this issue, but it looks like something is wrong in the sender portion of the emails. 

> On May 18, 2017, at 1:36 PM, Smith, Dennis P (IHS/HQ) [C] <[log in to unmask]> wrote:
> 
> I seem to be stuck between mail administrators pointing their fingers at each other for the issue of these probe failures.  I just do not know how I can get to the bottom of this issue and get it resolved.  I cannot determine if it is the mail server or the content filtering devices.  
> 
> The error is as follows:
> 	The reason for the problem:
> 	5.1.0 - Unknown address error 550-'REJECTED - Invalid characters in sender =
> 	email address [@]'
> 
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