Earlier quoted context omitted.
That's right. I use they/them. My question remains, though. True, it might be rare now, but it doesn't necessarily going to be rare in the future.
People accept they because it was an English pronoun already. They'll stop using he and she before they accept custom pronouns.
My issues is that it would be hard for me personally to write an Email to several different people with different gender pronouns. I was asking if there are alternatives or maybe even similar situations in other cultures/languages that might hint to a more general solution.
It seems I was not clear enough, hence the downvotes above.