why won't my email password stay on my mail account?

When opening mail to access my emails i have to renter my password as the IMAP server for yahoo will not accept the password. When looking at my account on mail the password is not stored in my account. Even if you input the password onto the account and save it, it will work a few times but then the password disappears again. I have reset my password with my email provider but still it does not work.

HELP.

Thank You.