Issue: Mail sent to Yahoo! addresses, getting results in a SMTP errors, as in this example:
SMTP error from remote mail server after initial connection: host b.mx.mail.yahoo.com [220.127.116.11]: 421 4.7.0 [TS02] Messages from 18.104.22.168 temporarily deferred due to user complaints - 22.214.171.124; see http://postmaster.yahoo.com/421-ts02.html
SMTP error from remote mail server after end of data: host e.mx.mail.yahoo.com [126.96.36.199]: 451 Resources temporarily not available - Please try again later [#4.16.5].
Despite the claims in the SMTP error message, there are no user complaints regarding either mail from the server or from the domain from which the mail is being sent. A review of the relevant newgroups, operator mailing lists, and spam filtering/association show no complaints regarding mail server IP or from the client domain.
Solution: In the meantime, there is no solution. Our server has configured with retry setting, hence deferred mail will return to the queue for periodic retry, as is the case with other mail that cannot be immediately delivered for any reason. The mail may be delivered, or may be deferred once more, this will depend on which of Yahoo's mail servers receives the next connection. This is controlled by Yahoo, and is not an issue we can address.
Additional notes: We have been unable receive any answer from Yahoo! as to what conditions exist that cause them to defer mail in this manner, even, with new IP, it is getting the deferred message as well. Deferrals have occurred on servers where mail activity is minimal at best, and where no automated mailings or forms exist, and on servers where there is high mail traffic, both in- and outbound. When contacted via their form, Yahoo often sends back (days later) a request for information that has already been provided to them, rendering the process useless and unnecessarily time-consuming.