Hi,
The below things have worked for me. Let this be input for anyone who is facing the yahoo temporarily deferred message. The steps below assume that you already have the knowhow of blacklists , blacklist removal, DNS configuration etc. If you do not have this knowhow - then you can google it up to learn.
1 ) Make sure that the sending IP address is not in any blacklist. Some blacklists like WebIron do not allow you to request removal. Be particulary careful of such blacklists. If your IP address is listed in a blacklist which does not allow removal request - then it is best to get another IP address.
2 ) Use a dedicated IP address ONLY. Shared IP addresses will not work properly for sending more than a couple of emails are any given point in time.
3 ) Ensure that your DNS is properly configured with A records for your server, TXT records for the mailing IP address and RDNS entry is setup properly. Your IP address MUST be FCrDNS compliant
4 ) Setup SPF & DKIM authentication on your server
5 ) Ensure DMARC policy is published on your server
6 ) Sign up for Yahoo's complaint feedback loop
7 ) If you have signed up for the complaint feedback loop, uyou will receive an email informing you whenever your email is marked as spam. If you receive this feedback - you need to remove that user from your mailing list - this is a MUST.
Needless to add that you must ensure that you have obtained it by genuine subscription. If you have bought an email list - it will most surely contain email addresses which are spam traps - dummy email addresses created by mail providers but never used by them to subscribe. If your IP address sends even a single email to a spam trap (even by a mistake of a mistyping of the email address) - you have had it. Your IP address will be blacklisted with no recourse to request removal.
In a nutshell - Follow good, ethical sending practices and your IP Address will develop a good reputation with Yahoo.
When that is done - HOLD ON TO THIS IP ADDRESS!!!
Happy emailing.