We have huge project with a great number of registered users. They receive notification via email, when actions occur. (new comment, gift, some other activity). Project targeting – Russia and former USSR In Russia some free email hosting providers like mail.ru delete user account after 2-3 years of inactivity.
So we have now 2-3000000 users with bad email address.
How to find them and remove from mailing list:

According to RFC 5321 (smtp rfc) in case of wrong email in rcpt to, server should return 550 no such user.
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