Let them mark self-hosted email as spam.
Just like the nostr notes I post will still be seen by readers who kept the thread (but perhaps filtered out of Primal one day), any self-hosted emails will be read by readers who remember what email is.
Discussion
I only had 1 email get marked as spam because the domain was too new. After that none of it got marked as spam by MSExchange. It is surprisingly still interoperable for send/receive.
(I have a similar policy on my own mailserver.)
Google kept marking my multiple-year domain as spam just because it was self hosted.