On 13/11/18 10:12, Brian Kantor wrote:
When I retired, I switched email servers from UCSD.EDU to my own domain, and mail from me was occasionally marked as spam by gmail. That gradually stopped. But mail from me as brian@ampr.org that was NOT in reply to a message I'd received was FREQUENTLY marked as spam by gmail. Now, a year later, that is rare. I believe this shows that Google is constantly adjusting its spam filtering mechanism, and that some part of that adjustment is automated.
Gmail users can help train the filter. Simply take any mail wrongly marked as spam out of spam, and Gmail will use this as a training input. If enough affected people do it enough times, the spam filter does eventually learn.
Microsoft's email service (with all the names it goes by) has a reputation of occasionally just discarding inbound mail with no notification to either the sender or the recipient. I have proven this with certain email contents such as long lists of IP addresses in the body of the message; the message just vanishes. Attachments seem to be better tolerated, gzip'd or bzip'd attachments seem to get through most of the time.
Another thing is simple scripted email posters, as commonly used by scripts are also often victims of Microsoft mail deletion. I proved this many years ago when clients of a training company I worked for at the time were complaining that they weren't getting booking information. It soon became apparent that all of the affected clients had a Hotmail address, and testing with Hotmail showed that their service silently deleted the automatic emails from our booking system.