I have a mail server:
That is work. But, how to configure my MAILSERVER so that MAIL CLIENTS (thunderbird, apple mail, outlook) can automatically find settings ?
In short - you can't.
Some clients will try to guess settings, they are not so very complex. GoogleMail client for Android need only hostname and user's credentials to set up connections. Thunderbird acts very similar.
But you can't give a hint to the mail client in any way, as far as it wasn't implemented by SMTP/POP/IMAP protocols.
This is not a matter of mail server configuration.
You could add specific SRV records to your DNS zone, that may be parsed by the email client :
_imap._tcp SRV 0 0 143 imap.example.net. _imaps._tcp SRV 0 0 993 imap.example.net. _submission._tcp SRV 0 0 587 smtp.example.net.
Here, "143" is the imap port you use, "imap.example.net" your imap server, and quite the same for smtp. You can also add pop3 and pop3s record if you still use it.
Some clients may also make a request using information from another DNS record "_autodiscover.tcp.example.net", so you could also add this record, and configure a web server to answer to this xml request. This is detailed in https://web.archive.org/web/20150817115525/http://moens.ch:80/2012/05/31/providing-email-client-autoconfiguration-information/, and there is a Github project that provide the php script and configuration : https://github.com/gronke/email-autodiscover
Mail clients sometimes guess hostnames, so if you name your hostname "mail" or "imap" it will probably work in most mail clients.
For thunderbird (only) there is a spec that Mozilla made up and never caught on, which you can set up, called autoconfiguration which will tell thunderbird clients where the mail server is.