How do I recover my incoming mail server password ?

If your password isn't saved in your keychain, there IS (or should be) no recovery option. No site should ever offer to send you your password, because if they can do that, it means they're storing it an insecure manner.

Instead, sites should offer a method to reset your password to a new password, after performing some kind of account verification (memorable question/answer; text message, etc.)

That said, the answer to your question 100% depends on your mail service provider. Since we don't know your mail provider we can't be specific, but I'd start by checking their support site for 'password reset' instructions.