"but supports OAuth" should be qualified. Neither FairMail nor Thunderbird support custom oauth configurations. Both only come with preconfigured oauth setups f

"but supports OAuth" should be qualified. Neither FairMail nor Thunderbird support custom oauth configurations. Both only come with preconfigured oauth setups for a small number of providers including gmail and outlook. It might actually be just these two. Perhaps one of them supports gmx or yahoo as well. That's it. If you have your own SMTP/IMAP with oauth support you cannot use either tool to log into it. The only actual software that I know which supports custom oauth configuration is Roundcube webmail. It also happens to work beautifully with postfix + xoauth. By the way, the last time I was searching for an email client with oauth support I got the distinct impression that oauth was on its way out anyway.