Есть маленькая вероятнсть получить подтверждение о доставке:
RFC2298:
2.1 The Disposition-Notification-To Header
A request that the receiving user agent issue message disposition notifications is made by placing a Disposition-Notification-To header into the message. The syntax of the header, using the ABNF of RFC
The presence of a Disposition-Notification-To header in a message is merely a request for an MDN. The recipients' user agents are always free to silently ignore such a request.
Также есть малая вероятность проверить наличие такого ящика на SMTP сервере:
RFC821 - SMTP протокол
3.3. VERIFYING AND EXPANDING
SMTP provides as additional features, commands to verify a user name or expand a mailing list. This is done with the VRFY and EXPN commands, which have character string arguments. For the VRFY command, the string is a user name, and the response may include the full name of the user and must include the mailbox of the user. For the EXPN command, the string identifies a mailing list, and the multiline response may include the full name of the users and must give the mailboxes on the mailing list.