Repeatedly getting failed email sends: "you weren't recognized as a valid sender"

Over the last week or so, one of my accounts have been getting restricted.. over and over again. Not sure why...

I have changed password

I have removed from the Restricted user list

Works for a while.. then I get the error again.

I have verified the emails going out are not anywhere near the limits of Exchange. We are at about 100-150 emails per day. 1-4 emails per minute (no every minute but when there are multiples, we do not get over 1-4)

It gets restricted again, but not in the Restricted list. I wait and it comes back the next day.

Works for a while, then I get the error again.

Go through the same steps, remove from Restricted list, check limits, etc.... Then it starts working again... then it stops... fix... works... stops.. repeat.

I have gone through the SMTP reports and I do not see any suspect emails going through.. verified again the limits.. I can't figure it out.

HELP!

PS. This account is used by our web based system and windows program to send reset emails, confirmation emails, etc. So the emails do look similar but there are differences in each email. Subjects are slightly different as well.