SMTP error from remote mail server after initial connection: host b.mx.mail.yahoo.com [74.6.136.65]: 421 4.7.0 [TS02] Messages from 110.4.45.93 temporarily deferred due to user complaints - 4.16.56.1; see http://postmaster.yahoo.com/421-ts02.html
SMTP error from remote mail server after end of data: host e.mx.mail.yahoo.com [67.195.168.230]: 451 Resources temporarily not available - Please try again later [#4.16.5].
Contrary to the above error message, there is no verifiable complaint regarding the IP or domain from which the email is sent. Checking public antiSPAM blacklists will show the lack of any IP or sender domain records.
Unfortunately, there is no solution to this problem. Our servers are configured to try again to deliver a rejected message at different time intervals. At the next delivery attempt the email can be accepted or rejected again. Yahoo! makes its own choices regarding the configuration of its servers, choices over which we have no control.
Note: After contacting Yahoo we were unable to find out the cause of these error messages.
