I agree that needs to be filed as a bug. PhillipTP please do that. If you wish, I can help verify details as I am dealing with this very problem for a group of Apple fans using a mail server running on the CentOS platform.

I have run into a similar problem with mail.app on MacOS 12 and iOS 15. Seems to be related to libreSSL breaking or tripping over local ssl certs. $ openssl s_client -starttls smtp -connect smtp.mydomain.com:465 [snip] 4346660396:error:06FFF089:digital envelope routines:CRYPTO_internal:bad key length:/System/Volumes/Data/SWE/macOS/BuildRoots/533514bb11/Library/Caches/com.apple.xbs/Sources/libressl/libressl-75.60.3/libressl-2.8/crypto/apple/hmac/hmac.c:188: 4346660396:error:140203FC:SSL routines:CONNECT_CR_SESSION_TICKET:sslv3 alert bad record mac:/System/Volumes/Data/SWE/macOS/BuildRoots/533514bb11/Library/Caches/com.apple.xbs/Sources/libressl/libressl-75.60.3/libressl-2.8/ssl/ssl_pkt.c:1200:SSL alert number 20 4346660396:error:140200E5:SSL routines:CONNECT_CR_SESSION_TICKET:ssl handshake failure:/System/Volumes/Data/SWE/macOS/BuildRoots/533514bb11/Library/Caches/com.apple.xbs/Sources/libressl/libressl-75.60.3/libressl-2.8/ssl/ssl_pkt.c:585:

I have a good use case for this. If Rob, or someone else, has made some progress, I would like to chat.