UPDATE and success: several very helpful readers helped me debug the issue of GMail putting my emails into the junk folder—many thanks. There goes my morning, but it was worth it.
Not only should GMail (or any other mail server) not mark my mail as junk, it also does SPF=PASS, which is an improvement over before where it was neither yeah nor nay, raising the hackles of spam filters. Now that is solved.
Receiving end (not me)
Sometimes subscribers supply email addresses that don’t work at all, which is an issue on the receiving side, nothing I can do about that. Actually it leaves me helpless to communicate with the buyer, since the email is bad and I get no phone number from PayPal.
Examples of bad emails on the receiving end:
The following message could not be delivered to ****@earthling.net at host earthling.net (18.104.22.168) because the host is not accepting mail.
The following message could not be delivered to ******@gmx.info at host gmx.info (22.214.171.124) because the host is not accepting mail. 554-gmx.net (mxgmx116) Nemesis ESMTP Service not available
DM3NAM03FT018.mail.protection.outlook.com rejected your message to the following e-mail addresses: *******@autonation.mail.onmicrosoft.com DM3NAM03FT018.mail.protection.outlook.com gave this error: Hop count exceeded - possible mail loop
<****@busmail.net>: host 127.0.0.1[127.0.0.1] said: 552 5.2.2 [MDA 1.3.0-22951] Delivery failed for <****@busmail.net> : User's mailbox quota exceeded (in reply to end of DATA command)