Check if a mail server accepts connections on ports 25, 465, and 587. Also tests STARTTLS and TLS security for all MX servers.
Enter a domain to test its SMTP server connectivity and TLS security
SMTP uses three standard ports. Port 25 is the original mail relay port for server-to-server transfer, but most ISPs and cloud providers block outbound port 25 to prevent spam abuse. Port 465 (SMTPS) wraps the SMTP session in SSL/TLS from the start. Port 587 is the current recommended submission port for clients - it starts plain and upgrades to TLS via STARTTLS. When a server accepts a connection, it sends a 220 banner identifying the server software and hostname.
STARTTLS allows a mail server to upgrade a plaintext SMTP connection to TLS. If not supported, server-to-server email is transmitted unencrypted and vulnerable to passive surveillance and active downgrade attacks. MTA-STS (RFC 8461) goes further by publishing a policy that explicitly requires TLS when delivering to your domain. Without MTA-STS, STARTTLS is opportunistic and can be stripped by a network attacker. With mode: enforce, senders that cannot establish TLS will queue and retry rather than fall back to plaintext.
Sent to our server: Hostname. We look it up in DNS. We connect to it. We fetch files from that site. Part of this work runs on our own server. To answer a repeated check fast, we have to keep answers for 5 minutes; then they are deleted. Also added to the page link, so you can share the result: Hostname.