check in mxtoolbox.com to see if you IP is in any of the blacklist spam database. If so, try clearing the IP by sending a removal request. Certain request like SBL will require the hoster / datacenter to send the email instead. So do take note of what time of ban you have.
Probably because you haven't set the SPF records on the DNS server for the domain name you are trying to send as.
Essentially, most of the big mail servers are now using what you specify in the spf records to know what IP address should send and the mx record server name.
Look up some SPF record tutorials and there are some wizards out there to help you create them.
Hope this helps!