UPDATE: FIXED as of 04/22!
A quick note for everyone, my web host is dealing with a malware attack that has compromised portions of their email servers with some sort of spam bot. My own email account is safe and not sending out waves of spam, but the attack caused my host’s email servers to get flagged on a spam blacklist that some ISPs (namely Verizon and Comcast) utilize. That means any email I try to send to @comcast.net or @verizon.com addresses, and possibly some others, will get kicked right back in my face as being blocked for “spam”.
I’m working with my host to resolve the mess now that I know the cause of the occasional email returns from the past few months. I’ve brought the malware to their attention and expect it to be solved ASAP.