When I try to send e-mail I get an error message stating that the e-mail address was rejected by the server. When I send to e-mail from another address, I get back an error saying the e-mail address is unknown. My e-mail has worked fine for years until

Question

When I try to send e-mail I get an error message stating that the e-mail address was rejected by the server.  When I send to e-mail from another address, I get back an error saying the e-mail address is unknown.  My e-mail has worked fine for years until yesterday.  HELP!

Answer 1

The specific text of the error message is very important here — I'm not sure exactly you're encountering here.
If you're able to connect to your email server and are able send email to other email addresses and if the failures are specific to one email recipient address, then please contact the intended recipient of the failing email, and confirm their address is valid.  (This is the way I'm reading your question.)
If you are unable to send any email to any other email addresses and this is specific to your email address, then try the web mail client interface (if one is available) to verify your login user and password, and check with your email ISP for assistance.  If your email password works via web mail, follow this Apple troubleshooting guide, then — if everything else fails — I'd probably then entirely remove the email account from Mail.app and re-add it per your email ISP's particular setup requirements.  (Some issue with the setup or maybe a corrupt setting in OS X or a problem at the mail ISP servers is a common problem, but this effects attempts to send to all email addresses via that account.)
An email account setup is specific to an email provider, unfortunately.   If you're using one of the more common email ISPs, then there are usually setup guides and frequently-asked questions posted online.