Hi Till (Thanks for the reply).
I did reconfig ISPConfig, CentOS network and everything else where I suspected some Upcase, but I end up with the same result e.g. web52_sale can send message (either from Outlook or WebMail) but I get the same error message when I send eMail to firstname.lastname@example.org
(showing lowercase thought) - see below (some french in the message but I'm sure you will figure the message). Attached are newer versions of my files...
Adresse du destinataire : email@example.com
Raison : Remote SMTP server has rejected address
Code de diagnostic : smtp;554 5.7.1 <firstname.lastname@example.org>: Relay access denied
Système distant : dns;mail.khameleo.ca (TCP|10.23.32.77|52990|22.214.171.124|25) (ns1.khameleo.ca ESMTP Postfix)
P.S. I really appreciate your help...