Hmm, this is all still a bit confusing to me. Sorry for my denseness.
I believe my mail client (Thunderbird) doesn't try to connect with SSL, unless my normal default setting "Use TLS, if available" is some form of SSL.
While I was able to reproduce the warning by temporarily setting "Use SSL" as an experiment, the warning also appeared before and after the experiment every 24hrs with my normal default.
No reboots, or TB restarts inbetween, so not apparent session relation. Of course, with my ignorance of such matters, a session may be time-based, for all I know.
Bottom line questions:
1) What would cause an SSL related certificate warning to appear if my client is not set to use SSL?
2) Is it safe to simply accept the mail.dreamhost.com certificate permanently, or might it have other effects I should know about?
Thanks for the rigorous discussion, folks. I appreciate the community.