退信訊息 RECIPIENT (relayed to non-DSN-aware mailer)

當收到退信的內容如下

----- The following addresses had successful delivery notifications -----
xx@xxx (relayed to non-DSN-aware mailer)

理由 http://www.sendmail.org/~ca/email/dsn.html#DSN

The first problem is addressed by DSN in the following way: if it delivers an e-mail to an MTA which doesn't support DSNs it will tell the sender so

應該是收件者的 Mail Server 不支援 DSN (回條??)