Recently we have setup a new SMTP server to send newsletters and promotional mails to our registered users. There are 5000 users who will receive newsletters on weekly basis. We are getting bounced mails, when we tried to send promotional mails first time. We have setup SPF records and domain keys as well. But we are not able to understand the reason why Yahoo blocked emails. Can you suggest how banks and other big websites send emails with out any problem?
Here is the content of bounced mail that was sent by Yahoo.
Could not deliver message to the following recipient(s):
Failed Recipient: firstname.lastname@example.org Reason: Remote host said: 421 4.7.0 [TS01] Messages from xx.xx.xx.xx temporarily deferred due to user complaints - 18.104.22.168; see http://postmaster.yahoo.com/421-ts01.html
Before we use new SMTP service, we used to outsource this work to third party email marketers and we did not configure SPF or domain keys that time. But it was successful. Which way actually they follow?