I have been dealing with this since a few years and this is a terrible waste of time. I believe there are way too much security on the Internet. Recently, I have discovered that an email I am sending to some customers about their renewal was put in the Junk E-mail folder of Outlook. When I discovered that, I have been trying to isolate the exact reason of this rejection. Before I had a subject like this:

"Failure to process your payment on the Universal Thread"

This was rejected.

I adjusted to:

"Failure to renew your subscription on the Universal Thread"

At first, I thought this was because of the word payment. But, it turned out this was not the case. That particular string was simply rejected as is. I really do not have know where is the source of this. But, this simply doesn't make any sense. If someone has any idea on why the first subject is rejected, I would appreciate any information on it.