Solved the Intermittent ATT/Yahoo Outgoing SMTP Mail Problem

After signing up for ATT DSL, I had intermittent issues sending email, where outgoing mail would infuriatingly take 2-4 tries to send. Downloading email was no problem. I finally figured it out.

Assuming you have correctly configured your outgoing SMTP server (port 465, SSL enabled, with your login and password), the problem lies with Yahoo's spam filtering and your local authentication cache.

1. First, you have to enable each of your mail accounts (in the "From" field) with Yahoo. This is done through the same process by which you can check those email accounts through their web interface. There are instructions for how to do this from Yahoo (link).
2. Next, if you are using the Mac Mail client on OS X, open Keychan and delete all password/certificate entries that are related to your SMTP servers. This is the crux of the problem... if your entries are expired, particularly if they were created before step 1, Keychain will keep the older entries.

And there you have it! I think many ATT DSL customers must run into this problem... hopefully they'll run into the solution on this wonderful blog.