Wherever your email software prompts for the email username, enter in your full email address (e.g., firstname.lastname@example.org, not just example).
Be sure you set your email for SMTP authentication. Search your software's help system for terms such as "SMTP", "authentication", "server authentication", etc. You may also review the instructions for the specific software in other FAQs above, to see if you can infer any similarities. If you cannot determine how to set your email for SMTP authentication, then you might wish to consult the software maker's Web site or contact its tech support.