EmailDomainResolvable
/**
- Verifies that an email address' domain can receive mail.
- Unlike Laravel's
email:dnsrule, this fails open on transient DNS - failures (SERVFAIL / timeout, where dns_get_record() returns false). It only
- fails closed when the domain definitively resolves with no mail-capable
- records, which catches genuine typos like "gmial.com" without rejecting
- legitimate addresses whose nameservers are briefly unreachable.
*/