Your email account might not be properly configured with Frontier email servers. To correct the errors that you may be receiving, you will need to configure your email client with an email address other than that which produced the error message. For example: email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com or firstname.lastname@example.org. The reply-to email address in your email client can be set to whatever you wish.
If that does not correct the problem, you must enable authentication to our outgoing (SMTP) mail server. If you are having difficulties with your Frontier email address, click here. If this is an issue with a non-Frontier email address, please contact their support team.