I'm trying to send a mail message using System.Net.Mail.SmtpClient
via Yandex Mail and have the following transcript:
S: 220 smtp17.mail.yandex.net ESMTP (Want to use Yandex.Mail for your domain? Visit http://pdd.yandex.ru)
C: EHLO my.domain.name
S: 250-smtp17.mail.yandex.net
250-8BITMIME
250-PIPELINING
250-SIZE 42991616
250-STARTTLS
250-AUTH LOGIN PLAIN
250-DSN
250 ENHANCEDSTATUSCODES
C: AUTH login Base64EncodedLoginHere
S: 334 UGFzc3dvcmQ6
C: Base64EncodedPasswordHere
S: 535 5.7.8 Error: authentication failed: Use Web to Login !
And when I change the password to an incorrect one I get 5.7.8 Error: authentication failed: Invalid user or password!
instead so it's definitely not because the username or password are wrong.
And if I try to login via web interface it works just fine.
What's wrong? Why do I get "use web to login" message?
Best Answer
Most likely it's because of Yandex anti-spam policy. They assume that everyone without verified mobile number assigned to their mail account is spammer and should be barred from using SMTP.