[darkengine]

Being able to send an SMS to a US phone number does not guarantee the ability to send an SMS to, eg, a Chinese phone number. SMS isn't even widely used in a lot of locales outside North America. Email works the same everywhere.

[usr1106]

Email is not delivered reliably either. There are many blockers. Either you are not allowed to enter a perfectly working address. They might use some list of domains they think they are bad (happens to me increasingly). The programmer might have hand-crafted their own regexp for "valid" email addresses. The email provider might not accept certain incoming mails. I am on several lists that say, sorry our emails are not deliverd on hotmail, yahoo, whatever. Harmless hobby stuff, nothing spammy or illegal.