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Connects to port 25 of the domain's primary mail server, reads the welcome banner, and checks whether it announces STARTTLS support. It does not authenticate or send any real mail.
It only opens the connection, reads the banner and capabilities announced after EHLO. It does not log in, does not send mail, and does not exchange credentials.
STARTTLS allows encrypting the SMTP connection in transit. Without STARTTLS support, the initial mail exchange between servers can travel unencrypted.
Frequently asked questions
No. The connection is closed with QUIT right after checking the server's capabilities.
Many hosting providers block outbound port 25 by default, and some mail servers only accept connections from known IPs.