Need help with an email error msg

Closed - This job posting has been filled.

Job Description

Some times when i send an email, i receive this error message:

Delivery to the following recipient failed permanently:

(Their email address)

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain (Their website address) by (Their website address) . [].

The error that the other server returned was:
550 SPF: is not allowed to send mail from (my email address)

I need this fixed as soon as possible.