Some step forward here....
Mystic use its code to send email trought an external SMTP, so is't not an OS/sendmail misconfiguration problem and then I was able to completely remove sendmail* from my OS.
On my firewall (in real time filter) I saw that requests on ports 587 and 465 are sent to theexternal SMTP server so the problem was certainly  authentication. I switched SSL/TLS from yes to no and changed port to standard 25 and it worked, then the problem is cryptlib related, i think; i use 3.4.5.0 compilend under Ubuntu server 22.04 and g++ 9 (is't the only way to compile it under my os).
g00r00 what do you think about?

--- Mystic BBS v1.12 A47 2021/12/24 (Linux/64)