After looking at this, I am pretty sure the problem likes outside our system. A partner of ours has been sending email to us and one particular user receives the email but cannot see the HTML version of the email.

Here is that users client information:

Click image for larger version. 

Name:	Screenshot-1 for Groupwise - 20170619.JPG 
Views:	2 
Size:	47.2 KB 
ID:	5835

His view option information:

Click image for larger version. 

Name:	Screenshot-2 for Groupwise - 20170619.JPG 
Views:	2 
Size:	55.8 KB 
ID:	5836

When he views the email the ability to view HTML is grayed out.

Name:  20170620-dalet_gw-02.PNG
Views: 61
Size:  14.5 KB

I scratched my head long and hard. I then looked at the firewall / SPAM filter and noted that the email going to the user that cannot read HTML is larger in size than the same email going to other users. For example an email I had the partner send to myself and two others including the person who cannot read the HTML version (dale.thomas@mhtn). Here are the firewall entries for that email.

/var/log/smtp.log:2017:06:21-12:05:10 firewall smtpd[8595]: SCANNER[8595]: id="1000" severity="info" sys="SecureMail" sub="smtp" name="email passed" srcip="104.47.42.56" from="jack.chin@hok.com" to="dale.thomas@mhtn.com" subject="FW: Overdue/Urgent: RFIs/Submittals" queueid="1dNk06-0002Ed-EM" size="272096"
/var/log/smtp.log:2017:06:21-12:05:10 firewall smtpd[8601]: SCANNER[8601]: id="1000" severity="info" sys="SecureMail" sub="smtp" name="email passed" srcip="104.47.42.56" from="jack.chin@hok.com" to="james.knight@mhtn.com" subject="FW: Overdue/Urgent: RFIs/Submittals" queueid="1dNk06-0002Ej-I6" size="212591"
/var/log/smtp.log:2017:06:21-12:05:10 firewall smtpd[8601]: SCANNER[8601]: id="1000" severity="info" sys="SecureMail" sub="smtp" name="email passed" srcip="104.47.42.56" from="jack.chin@hok.com" to="daniel.wells@mhtn.com" subject="FW: Overdue/Urgent: RFIs/Submittals" queueid="1dNk06-0002Ej-I4" size="212591"

Note the size of the email is 59,505 bytes larger. The sending system is Office 365 / Outlook.com.

My question is what would cause the email to be larger of this one user? Am I barking up the wrong tree to think that this might be the problem?