For the old account, since you're using 25, and authentication, it sounds like you don't need a password at all. There might not even be one set for the SMTP server. But, try 465 + TLS + auth=plain again and check the log. It probably won't help though.
You can try changing the SMTP server to one of the other accounts' SMTP server, use 587 + TLS and see what happens. But, I doubt that'd work.
You'll probably have to use wireshark to capture your network traffic to see the incoming server traffic for the old account. Depending on how you connect to that account, (port 143 + auth=auto + TLS unchecked for example), you might see your password in the raw data. Or, it might be base64-encoded like in the log. If using TLS though, you'd have to look up how to decrypt SSL traffic with wireshark to see your password.
The alternative is to use the "forgot password" feature for your email provider and or contact them.
