Hi, is it asking for the Keychain PW, or the PW for that Mail account?
Keychain Access asks for keychain "login" after changing login password...
If the later, check for duplicate Keychain entries, there are normally 2 for each Mail account, one in & one smtp.
Help, I forgot a password in my Keychain
Open Keychain Access in Utilities, use Keychain First Aid under the Keychain Menu item, then either check the Password under that item, change it, or delete it and start over.
Resetting your keychain in Mac OS X...
If Keychain First Aid finds an issue that it cannot repair, or if you do not know your keychain password, you may need to reset your keychain.