How can I set a consistent font for sent messages in Apple Mail Mojave? macOS Mojave Mail still ignores a body font for outgoing emails. Rich Text emails show up with Times New Roman font on Windows Outlook clients. Mail does not set a font type, Thus, Windows Outlook uses its default paragraph formatting which falls back to Times New Roman. It's so unprofessional and embarrassing

Using plain text seems to be the only solution. Big downside: No bolding, no highlighting. This has been an ongoing issue since years. There used to be a great plugin called Universal Mailer which is no longer developed. 

Any workaround for Apple Mail Mojave and Catalina? I highly appreciate any solution.
People who send plain text mails are heroes to me. There’s no downside there since it respects everyone including screen readers, people who have a font and size preference, etc...

There is a real cost to rtf bloat and html and you are seeing some of that pain here.  My solution is to eschew bu to know some people look down on that as plain text can be rendered horribly as well by some Mail clients. 

Sadly, you have to read the room for your intended audience and choose an authoring tool that is copacetic. A third party mail app might be needed if no one has a style tweak that lets you keep the styling features you know you need to communicate effectively. Outlook on Mac does a decent job of hiding that the mail came from Apple if you have the desire to try that.