“Sir or Madam” already is a “They/Them”.

Sadam.

I’d guess “To whom it may concern” would be the most gender neutral/inclusive way to rephrase that.

“Dearest snookums”

Not that I know of. Unless someone has stated their gender or pronouns, I just use people’s names and avoid anything that assumes or implies gender.

If you believe my therapist friend who is also non-normative in a relevant fashion, demands for pronouns are often ‘othering’ in result, and thus go from farce to force.

So, I write fewer email.