The mail server rejected your email from your webserver. A quick google search for the error code reveals:You are sending emails to an external domain and you get a Delivery Failure Message which says: 554. 5.7.1.
CauseSMTP Code 554. 5.7.1 is basically a rejection from the Recipient Domain. SMTP Code 554. 5.7.1 means 'Not allowed'. The address in MAIL FROM appears to have insufficient submission rights, or is invalid, or is not authorized with the authentication used; the address in a RCPT TO command is inconsistent with the permissions given to the user; the message data is rejected based on the submitting user.
It looks like your email server wont relay mail for @gmail.com domain.