decrypt incoming mail server password Apple Mail
In nearly all configurations, the sending and receiving password are the same.  If the incoming password shows as set in MacOS Mail, then it is in your keychain (because that is where those passwords are shown)

Go to "Keychain Access". tool in the Utilities folder of /Applications and click on "Passwords" in the left sidebar. This will show all of the stored passwords to you.  You can search for the mail server hostname in the top right search bar to narrow down the choices.  Once you see it, double click on it to open the details and click on the "Show Password" checkbox.  Once you supply your login password, it will show you what that password is.

After that, it is a simple matter of adding that password into the Account settings within MacOS Mail.

Good Luck!