Dedicated Server email problems

I have a problem our server seems to have ben blacklisted although we do not seem to know where from, but when we send email to gmail and company email address we get this error: SMTP error from remote mail server after initial connection: host mail. 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means. ------ This is a copy of the message, including all the headers. ------ ------ The body of the message is 142412 characters long; only the first ------ 106496 or so are included here. and when we send to gmail: The address to which the message has not yet been delivered is: Delay reason: SMTP error from remote mail server after end of data: host [login to view URL] 421-4.7.0 [[login to view URL] 15] Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited. Please visit 421-4.7.0 [login to view URL] to review our Bulk 421 4.7.0 Email Senders Guidelines. jc4si515528pbd.35 - gsmtp accoring to gmail guidelines we can creatre a buisness account and send that way?? fine if need be we just want to be able to send emails from our dedicated server withut any problems, as far as we are aware no spam emails have ben sent as when we look in the mail que they are just emails we have sent or system emails. hope someone can reslove.