OK, same, same really in MS Outlook there is probably something to use secure logon to the email server, that would mean Outpost creating a secure connection and the Mail Shield being locked out of the process.
Can you post the full text of the error window or attach a screenshot of just the error window (additional Options in the Reply window) ?
You should have two Yahoo.com entries in the SSL Accounts, as in my image (click to expand), are they the same settings ?
As far as uninstalling goes you could simply elect not to have avast email, but that isn't ideal by any means as I don't know if btinternet, a.k.a. Yahoo scan their email on the servers.