lalf@ubuntu:~$ telnet localhost 25
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 testsrv.com SMTP Guerrilla(1.5.1-90-gf9c4965) #1 (1) 2019-08-04T13:40:52+03:00

Why explicitly show the client that this package is used? Why not pretend Postfix?
Just sooner or later, the smartest system of sending letters will begin to filter domains with such an answer without even transferring any data further.

Maybe it makes sense to change this behavior? For example, make the setting in the configuration file.