mu228 The precise address of the mail server is included in the email with the service data as well as in the information on the hosting account panel, so it is not difficult to access knowledge. However, regarding "X," you haven't familiarized yourself with the help page at https://docs.serv00.com/Mail/
"The address of the mail server depends on the number of the hosting server. For the server s0.serv00.com it will be the address mail0.serv00.com, for the server s1.serv00.com - mail1.serv00.com, for the server s2.serv00.com - mail2.serv00.com etc. In the examples the address mailX.serv00.com was used."