mail pop3 password won't update

I use POP3 to download my email from my ISP to the OSX Mail Client, and needed to change my POP3 password after a virus threat.

While I can successfully change the password in Mail -> Preferences (for POP3), my my Apple computer is still sending an out of date password out when trying to log in - my ISP can see that.

Hence it looks like OSX is not accepting my password, even through it successfully accepts it in Mail preferences.

I hear there is something called Keychain - and I can see that is says my pop3 password was last set some weeks ago - so I can see it hasn't accepted my latest password change - but - I haven't a clue that Keychain is, and how to affect it to enable my latest POP3 password.

I'm not using iCloud - at all - so if it requires enabling that, I can't go there (iCloud, in syncing two of my devices before, wiped dozens of critical "Notes" from my iPad, so I just can't go though that again).

I wonder if anyone has the expertise and knows enough about this to advise me on how I can get "Keychain" on my computer to be updated with the latest Mail POP3 password I've set?

thanks,

Kevin Nolan.

(Dublin, Ireland).