We've recently had a phase, I'm not sure how long but of the order of a week or two, in which Yahoo! was bouncing StackCP mail client emails, with an error something like the following:

Reporting-MTA: dns; web-relay4.stackmail.com

Action: failed
Final-Recipient: rfc822;xxxx.xxxx@yahoo.com
Status: 5.0.0
Remote-MTA: dns; mta7.am0.yahoodns.net
Diagnostic-Code: smtp; 421 4.7.0 [TSS04] Messages from 45.8.227.179 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see postmaster.yahooinc.com/error-codes : retry timeout exceeded

The 421 SMTP error is documented on this page  SMTP Error Codes | Sender Hub .

Correct me if I'm wrong but this has resulted from excessive spam being sent from Stackmail (the email service used by StackCP) which Yahoo! has responded to by bouncing mail from Stackmail.

This is only a temporary state of Yahoo! affairs (emails sent to yahoo email addresses are now being received), however it does mean that email receipts for payments for ramble coaches were not sent for the duration that Yahoo! were blocking Stackmail, so it is not a trivial matter.


GSO