Hartmut:
Your case would be slightly different since you are getting drops, not
bounces. Your case could be more related to what I'm going to present
than Montana's.
Indeed, I want to attend, if I can get a ride. Otherwise, I'll try virtual, though that doesn't work well on my hardware (read: dated).
All the Big Email companies are making life extremely
difficult for little senders and huge senders alike. The Borg demands
its sacrifices!! :-)
That's what I've been preaching to my friends and customers for the last number of years, and I've been supplying them with e-mail facilities that do SPF/DKIM/DMARC properly. That still doesn't entirely stop Gmail (as receiver) from deciding by their divine algorithms to "silently discard" messages - not spam, no non-focussed (er, misfocussed) Inbox, no bounce, just silence - sender and receiver are none the wiser.
Hartmut