I just started off doing email marketing for some of my clients. I got myself a VPS and started sending out bulk emails for people (all gathered from newsletter signups). Everything was going great, then yesterday I was unable to send emails. Contacted my VPS host, they said everything is fine. Then I called up my ISP, they said everything was fine too. So I tried using my laptop to send emails using my VPS and it worked... I have not installed any anti virus / firewall software, etc... so I have no clue, if anyone has any insight, it would be much appreciated.