Knowledgebase: SmarterMail
Yahoo bounce message: 421 4.7.0 Temporarily deferred due to user complaints -

This error indicates Yahoo is seeing unusual traffic from your IP address and/or certain emails from our shared email server are generating complaints from Yahoo Mail users.

Yahoo users have been using the "Spam" button in their accounts too often in a short amount of time. Because of this, Yahoo is temporary blocking email either based on your IP address or our SMTP server address.

Did I do something wrong?

Perhaps (if Yahoo is referring to your IP address, yes), but it may also refer to our SMTP server IP address; in that case it may be another user that has caused this temporary block. If you have been sending regular mail, it is most likely our SMTP server that is temporary blocked instead of your personal IP address.

How can I fix this?

Yahoo encourage user to retry sending emails 4 hours after encountering the error. This is a temporary problem that will automatically get fixed over time. Usually these type of problems are solved within one or a few hours, unless the user complaints continue.