Bonjour,
1 - Is it normal and logical that in the email propreties, a customer can read our server-name, and not only his domain name ?
Yes, this is normal.
2 - Is this 'Undisclosed-Recipient' also normal and logical ?
It means they did not enter anything in the TO: line. If you use BCC you should put yourself in the TO line so you do not see Undisclosed-Recipient. That only means that no one was written in the TO line.
3 - Can we take any action, to make that only our customers domain names is showing in the email propreties ?
Someone else may know, I do not.
Many thanks for any answer
Bon soir