Hey everyone. We have a small app that sends transactional emails (order confirmations, password resets). It's been running fine for months, but since yesterday afternoon, Yahoo is suddenly rejecting our connections with a `421 4.7.0 [TSS04] Messages will not be accepted for this account`.
Our setup is:
* We use a dedicated EC2 instance as an SMTP relay.
* Postfix is configured to use our domain's DKIM and SPF records.
* We send via a dedicated IP that we've had for a while.
The error seems to be a temporary failure, but it's not going away. I checked our IP on a couple blacklists and it's clean. Our domain's reputation seems okay elsewhere (Gmail, Outlook are fine).
Here's the relevant part from our Postfix `main.cf`:
```smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, reject_unknown_helo_hostname
smtpd_sender_restrictions = permit_mynetworks, reject_non_fqdn_sender, reject_unknown_sender_domain
smtpd_recipient_restrictions = reject_unauth_destination, permit
```
Could this be a Yahoo-specific throttling issue because of a sudden volume spike? Or is there something obvious in our config we're missing for Yahoo's requirements?