This could be a good candidate for porting to Haiku.
The current email client options all have their own individual issues.
The default Mail app for Haiku isn’t really suited to large inboxes containing more than a few hundred emails and hasn’t worked for me at all with IMAP addresses.
MailNews is quite dated and the program hasn’t been patched in years, potentially leaving it open to exploit. Issues surrounding the password manager that have come to light recently would also not be patched in this old version.
Troljita looked like a good candidate but the development on the multi account support has stalled and doesn’t appear to be developed anby more.