I've got # in my address and some systems fail with it too. It's totally valid. As example outlook.com won't allow sending email to addresses with # . Also some

I've got # in my address and some systems fail with it too. It's totally valid. As example outlook.com won't allow sending email to addresses with # . Also some services reject short addresses for no reason. As example m@em.mm or so, that could be totally valid. You don't know it before checking for MX info and asking from the SMTP server if the address is valid in rcpt to: .