You can detect mail server using this tool.
The SMTP "EXPN" command allows you to expand a mailing list or alias, to see where mail addressed to the alias actually goes. For example, many organizations alias postmaster to root, so that mail addressed to postmaster will get delivered to the system administrator.
An attacker might use the disclosed information to harvest specific security vulnerabilities for the application identified.
Only use components that do not have known vulnerabilities, only use components that when combined to not introduce a security vulnerability, and ensure that a misconfiguration does not cause any vulnerabilities.