Are you using the right SMTP server? (Your ISP will have given you details of the server you should use to send emails. An example might be "smtp.example.com") If you are not using this the recipient might detect that the email was not sent using the account indicated in the email.
Check your smtp setting in your email client. In Outlook, click on TOOLS > ACCOUNTS. Then select the account and click PROPERTIES. Select the SERVERS Tab. Check the field headed "OUTGOING MAIL (SMTP)"
Pike
April 2004