Read If you need to update your keychain password on Mac - Apple Support. Refer to the last paragraph.
If you are no longer able to send email messages because you do not know the password for that account, the solution is to remove the affected email account(s), and add them again according to the numbered steps at the conclusion of If you can’t send or receive email on Mac - Apple Support.
If you do not know the passwords associated with your email accounts, then you will need to contact your email service providers for help. They all have different ways of confirming your credentials.
In any event if you no longer know the password for your Keychain, then that Keychain is no longer useful to you and you might as well delete it. Not a problem, but you will need to provide your credentials for all the services that use Keychain for credential storage and retrieval. You only have to do that once for each service.