Adil,
I might be wrong but feel you mix up "internal" and "external" services.
You have to distinguish between "internal" and "external" because you can do almost everything within you internal network as you manage DNS, IP addressing etc.. while "external" services are potentially restricted depending on what your ISP provides.
Looking at "internal" services only (and focusing on mail obviously):
- user's mail client has to be configured to point to Zentyal mail server (either POP or IMAP) and also use Zentyal as SMTP server.
- users need to be configured as Zentyal users with mail service enabled.
From there it should be straightforward for them to send/receive mail "internally".
Sending mail to internet "may" work too assuming your ISP doesn't block port 25 to mail servers not managed by your ISP (this is often done to limit spam and avoid ISP's addresses to be blacklisted. This can be solved by relaying mail via your ISP MTA.
Problem, once mail has been sent to internet using your @mytest.fr, is to receive replies :-)
it's very unlikely that you have spend money for such "test" domain. You should better use real domain and this cannot be done "for free", at least in France. You must rely on registar that will handle this request and set up domain for you on their own DNS. Then, as Christophe just explained, your DNS zone will contain MX record pointing to SMTP server. This can be any SMTP server configured to receive mails for your domain. If you have fixed IP address, Zentyal can be used (this is what I do
) otherwise mail will be delivered somewhere else and retrieved by Zentyal.
So, to make a long story short, before we can help you further, you have to tell us more about your preferred scenario
does it help