Hello Ray-Monde,
I am not an expert either but this seemed to work for me.
First step: Go to the Finder and click on applications
Second step: Find Keychain Access, click on it, probably have to enter your password.
Third step: go to the login on the left and click on it.
This will show a list of passwords that it is storing.
I found ones from the last 5 years that I know I didn't use anymore and deleted them.
I highly recomend seeing what the password is before you delete it. This way if you still need it you have it.
Fourth step: Click on Local items and do the same thing that you just did in the login folder.
Then you can exit the Keychain.
If you have removed the passwords for your email program it will ask you if mail can access your Keychain.
This requires your password for Keychain
Enter that and it will take your password from your macmail and store that.
Then I would quite mail and reopen it to make sure it works. Send yourself an email to test.
Good luck