When sending an email out from your Netfirms hosted address, the message may be bounce back with the following similar error:

The above message indicates that the recipient mail server is not accepting the mail due to a SENDER CALLOUT technique they are using to try and verify the sender (to see if the email address really exists) before accepting mail from the sender's domain. Because SENDER CALLOUTS can become abusive, especially by spammers, many mail servers including the ones hosted at Netfirms do not respond to SENDER CALLOUTS by disabling VRFY.