I've frequently been getting my mail returned to me, along with this diagnostic code the last few weeks.

There has been nothing in the emails I have sent that could even remotely be classified as "suspicious content", just regular emails to friends, so it's incomprehensible why these messages are coming. 

Occasionally -- about once every couple of months -- I send out a newsletter and am careful not to exceed the 100 recipient level. I only have a couple of hundred recipients total. My last newsletter was successfully sent out at the end of October -- to about 90 recipients -- and it was after that that these messages started appearing. The last time I had sent out a newsletter was around July/August, again only to about 90 recipients.

What can I do to restore the reliable functioning of my email address?

Thanks