I guess ... not many, but waaay more than "none"
I have a ...me.uk email address and still suffer from both of those problems. (The rest of the address is just lower case letters, dots and hyphens, so I'm assuming that some fool is complaining about the domain. Perhaps we should ask t.berners-lee to see if he's ever had problems with his amazingly unusual name. Oh, and as I write that, yes I'm thinking now of our regular commentard with the name O'Brien who is, if you'll forgive the phrase, "beyond the Pale".)
Happily, raised eyebrows amongst otherwise-tech-savvy colleagues doesn't stop anything working.
Sadly, email "validation" code does. Perhaps we need to send that memo round again:
You (yes, you, personally) cannot validate an email address. Every time you write code to try to do this, it costs you time to do it, time to deal with the customer complaints, and lost customers from those who can't be bothered to complain and just take their money elsewhere. There is no business case for trying to validate an email address. It just makes you look like an idiot when it goes wrong (as it will, see above). Stop it, you cretinous fuckwit. Go back over your life and remove all such checks from code you have written in the past.