I have no idea if it actually is, but it might be a legal issue too in some jurisdictions. German law for example requires commercial sites to have an option for quick and electronic communication - Email is generally a valid choice. If your mail server rejects incoming mail (be it because it's overly sensitive, offline, or you never bothered setting it up etc), that could very well be considered as non-compliant.
Yes, but that has it's own challenges with spam protection, privacy laws, users not spelling their email correctly etc, and you still need an email address to receive those messages (or check for new form entries regularly), so an email address is often used as a shortcut.
I have no idea if it actually is, but it might be a legal issue too in some jurisdictions. German law for example requires commercial sites to have an option for quick and electronic communication - Email is generally a valid choice. If your mail server rejects incoming mail (be it because it's overly sensitive, offline, or you never bothered setting it up etc), that could very well be considered as non-compliant.