Polarbar 1.2.5e was quite stable with Java 1.3.1 and some font fixups. As stable as Thunderbird, anyway.
It's a lovely mailer, and I'm sure once you use it, you won't be able to go back to Post Road.
IMAP didn't work perfectly, and I don't use POP much anymore, so I was hoping to run the new version. Maybe if BWW ports Java 11 or newer, this could replace Thunderbird for my use.