I have an
IMAP account on a private server and they said me to configure my mail with a
STARTTLS secure connection on the port 993 and authentication with standard password.
How can I set
I haven't tested this combination, but you should be able to enable it by creating the account as normal (be sure to use IMAP for mailbox access), then going into the Mail application's preferences -> Accounts section -> your account in the list -> Advanced tab, then setting the "Port" entry to 993 and selecting the "Use SSL" checkbox.
A couple of notes may clarify what's going on here:
The second point is the only worry I have here. Port 993 is normally the special port used for the "imaps" service, the go-straight-into-TLS version of imap. But in this case it sounds like the server provides plain-then-STARTTLS service on port 993, which might confuse Mail.app. I suspect it'll work fine, but I don't have a similar server to test with, so I can't be sure.
TLS is NOT the new name for SSL. It's the new version of SSL. SSL had some serious security problems and then TSL was developed. There were some security issues with downgrade attacks from TSL to SSL, so they had to turn off the downwards compatibility to solve these issues
SSL 1.2 != TSL 1.2