SMTP issues aren't uncommon, especially if you're using annoying ass Hotmail/Live/Outlook (Microsoft really loves to block any possible spammy emails from coming through to your junk folder). We had a user that couldn't even get an activation email and I was able to when creating a new account to test with soon after. Admins would also need to check XF logs, especially for SMTP-related issues, if there are any to begin with.