I am writing the solutions here because I also encounter the problem and cannot find any solution online.
By tracing the source code, I realize that the OSticket uses Pear Mail library to send email.
There is a conditional testing in pear mail socket library causing the problem.
When a user type in
eg. smtp.ionos.de, it translate into IP address eg. 18.104.22.168
due to the defects of the checking, it intends to do the reverse but yet there is no update or fix for that library.
So, to overcome this, for SSL port 465, one may type in ssl://22.214.171.124 with protocol
Because it takes any '/' and will use the hostname without trying to resolve it into IP.
It works because at the last moment before sending out the command, it will add the protocol anyway for connection purpose.
Hope this could save someone else's time from trying.