I just tried a test email and was surprised that it returns valid, even though a very basic self written test would identify it as invalid, as the mailbox is not available.
Why does't this easy one fail and report accordingly?
Array (  => email@example.com ) Array ( [mx01.t-online.de] => 10 [mx02.t-online.de] => 10 [mx03.t-online.de] => 10 [mx00.t-online.de] => 10 [t-online.de] => 0 ) try mx01.t-online.de:25 <<< 220-mailin87.aul.t-online.de T-Online ESMTP receiver fssmtpd2025 ready. 220 T-Online ESMTP receiver ready. >>> HELO test123456.de <<< 250 Ok. >>> MAIL FROM: <firstname.lastname@example.org> <<< 250 2.1.0 Sender accepted. >>> RCPT TO: <email@example.com> <<< 550-5.1.1 user unknown 550 5.1.1 Unknown recipient. >>> RSET <<< 250 2.0.0 Ok. >>> quit <<< 221-2.0.0 mailin87.aul.t-online.de closing. 221 2.0.0 Closing. firstname.lastname@example.org invalid.