Comcast SMTP Error Code 554. I started getting these when I moved my domain to Linode. Turns out that since Linode is fully IPv6 capable I ended up connecting to a Comcast mail server via IPv6. That mail server (and this error) comes from not having had a reverse-lookup DNS AAA record for that IPv6 address. Added that in, set it as a reverse pointer (PRT) record and it was all set. Took about 5 minutes to fix.