Problem Solved. Even though my service provider told me at my first inquiry that port 25 is open, I was not able to send mails. So after repeated searches through this forum, and almost taking my server apart, I called my service provider, and had a long chat with the support engineer. I told him that I have changed anything on my server, or my network, so he promised to get back after some time, well today I received an email from my Isp revealing that my IP has been cleared from any and all kind of blockages. Seems like they were filtering traffic, and though the port reported as open (checked through various online tools) legitimate calls to servers were being blocked. Anyhow, my SME is back online. Thank you for all your support.