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 do

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.