SMTP check
SMTP server check
We connect to the mail server of a domain (the MX with the lowest preference) and look at its banner, extensions, STARTTLS and certificate.
No mail is sent: the dialogue is fixed — greeting, EHLO, STARTTLS and QUIT. If the domain has MX records, the one with the lowest preference is checked; otherwise the host you entered. Internal addresses and this server itself are not checked. Some hosting providers block outgoing connections to port 25.