Got it! I had to set these three last values: postconf -e 'mydomain = sharingcenter.eu' postconf -e 'myhostname - mail.sharingcenter.eu' postconf -e 'myhostname = mail.sharingcenter.eu' postconf -e 'mynetworks = 178.63.65.136' postconf -e 'mydestination = $myhostname, localhost.$mydomain, localhost, sharingcenter.eu, mail.sharingcenter.eu'
I thank you guys for your patience and help. I just spent a good few hours googling today and working my way around blogs, documentation, howto articles, forum posts, mailing list archives, and the like. I wouldn't have even known what to google for without the patient and helpful assistance I've received here. When it is said that CentOS is a "Community ENTerprise Operating System" be there no mistake!
Cold beer for anyone visiting Israel soon!