Yahoo is the the most problematic email provider in the US. Its the most hacked and has the most issues with Mail clients like Apple Mail.app
Besides removing redundant keys related to email account that has trouble connecting the server. I found this Apple KB article that might help:
Mail (Mountain Lion): If Mail keeps asking for your keychain password
If Mail keeps asking for your keychain password, your keychain may be set to lock after a certain amount of time or when your Mac goes to sleep.
You can change the keychain lock settings to reduce the number of requests for your keychain password.
- Open Keychain Access.
- In the sidebar, click All Items in the Category section, and then select your login keychain.
- Choose Edit > “Change Settings for Keychain ‘login’.”
To reduce the number of requests for your keychain password, deselect the “Lock after [number] minutes of inactivity” checkbox, or increase the number of minutes. You can also deselect the “Lock when sleeping” checkbox.