You can detect mail server using this tool.
StartTLS is a protocol command used to inform the email server that the email client wants to upgrade from an insecure connection to a secure one using TLS or SSL. StartTLS is used with SMTP and IMAP, while POP3 uses the slightly different command for encryption, STLS.
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.