A company with Comcast Small Business router and Godaddy package for email finds that SOME of their emails to SOME people bounce back with "return codes" (usually 554 or 550). The "return code 550" bouncebacks happen even when the addressees are people who have been emailed hundreds of times successfully. Problem happens with webmail as well as mail clients (Outlook, Mac Mail with recommended server/port settings). Recipients have checked their spam folders.
When they send to my yahoo acct, it always goes through. When they send to my gmail acct, it bounces back to them SOMEtimes. Usually it's when other emails are forwarded/attached.
After great trial and error I found that, in ONE case, if they send me a NEW email msg that contains nothing but the plain text string "believeadjust.us" in the body, it will go to my yahoo but bounce back from my gmail. (If they delete the first "b" it doesn't bounce back.) The bounceback messages say:
Reporting-MTA: dns; p3plwbeout18-05.prod.phx3.secureserver.net [220.127.116.11]
Received-From-MTA: dns; localhost [18.104.22.168]
Arrival-Date: Sat, 07 Feb 2015 00:56:29 -0700
Final-recipient: rfc822; firstname.lastname@example.org
Diagnostic-Code: smtp; 550 5.7.1 more information. e10si13050465pds.193 - gsmtp
Last-attempt-Date: Sat, 07 Feb 2015 00:56:29 -0700
I also have full message headers from yahoo I can upload if nec.