I’m pretty sure that we were all taught to send emails in primary and JHS. We were also taught how to sign off. Professionally speaking, the things that we were taught are right. What has been used for the longest time is “All the best”, “All my best” or even “Best wishes”. I don’t know about you, but reading email sign-offs like that just feels a bit forced.
Even “Sincerely” doesn’t sound quite so sincere. It sounds taught. That being said, you can’t very well end your email without a sign-off. And there’s no way you’re ending a professional email with “XO” or “Love”.