How do I change the mail password on an iMac with Catalina 10.15.7. I receive mail but cannot send mail

Did this just start happening recently? Were any changes made? You have two options. (1) Open the email app and carefully check the user Id and passwords listed there to ensure that they accurately match the required data on the email service provider's web site. I would reenter in any case. (2) If that doesn't work you can move any saved email messages to a local folder, then delete the email account and create a new one. Hope the works for you.