Outlook Email body coming through as a Text Attachment from one domain but not another

Outlook Email body coming through as a Text Attachment from one domain but not another.  I have a client that uses EarthLink to Host their Domain and email. They have created an email that forwards to a specific group of email addresses (within the domain)... a little over a month ago, some of the customer emails started coming through as attachment.txt instead of showing the text in the body of the email. It is NOT all senders. Just a handful.

Originally, we thought that the customers had changed something on their systems that generated the emails. To test this, I created an email using a cox.net account and added it to the FORWARD. I added the email as an additional account in Outlook and monitored the emails coming from the customer. When email comes in from the forward, the email account hosted by EarthLink, still has a blank body and an attachment.txt. The Cox email account comes in with text in the body of the email.  ALSO, note that I can go to the EarthLink Webmail site and look at the same email and there is TEXT in the body of the email. I am not sure is this is an EarthLink issue or an Outlook issue or something entirely unrelated.   Other items to note... both accounts are POP3 and each is a separate PST account but outlook opens them in the SAME profile. I have even made sure that incoming and outgoing ports are the same (IN: 995/SSL and OUT: 465/SSL)  

If this was an Outlook issue, wouldn't I experience the same issue in both accounts? If it was an EarthLink issue, wouldn't there be a Text attachment in the Webmail interface?

The Email Forward is not the issue, because some users are now getting emails directly and there is only an Attachment.txt instead of Text in the body of the email...  This is not One user , but 25 users with two Office versions (2010 and 2013).   All users are using TrendMicro WorryFree Business Security for AV/Internet Security.

Where do I look? Is there a solution?
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Barry GillChief of StaffCommented:
Sounds like there could be an errant line in the message headers.

In the RFC's, the headers are separated from the body of a message by a blank line.

I have seen a blank line going into a message before the body starts, cutting off the headers early so that the body parts are not properly defined. This is almost never picked up by Microsoft Exchange recipients but once the message passes through other MTA's, this can become more prominent.

Are you seeing SMTP encoded stuff? Mime parts defined, attachments as long strings of text, HTML rendered as text?

If you are, have a look at what plugins are installed on the sender's outlook, what apps are interacting with the mail on your servers. This is almost exclusively the fault of the sending server/AV (well the fault I describe anyway and I have seen Trend Micro IMSS do this :) )
Dubbi47Author Commented:
Still working with our ISP . There has been no solution.
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