SOLVED: Postfix - Lost connection while receiving greeting
You put me on the right track. Since our mail server is on a private address, I could not verify whether my IP address was blacklisted with the site you mentioned. So, after investigation with my ISP provider, I found out that they have 2 mail relay servers one on address 10.XXX.XXX.49 and the other on 10.XXX.XXX.50. I figured out that I could not authenticate on the '50' one but it was OK on the '49' one. So, instead of having 'relayhost = Blank' in /etc/main.cf, I added relayhost = 10.XXX.XXX.49 thus forcing mails from my server to relay through address '49'. Now, mails are flowing without problem.
Thanks a lot for your help.