SMTP diagnostics
Check a mail server's banner, STARTTLS support, and open-relay exposure.
This SMTP test connects to a mail server and reports what it finds: the greeting banner, whether STARTTLS is offered for encrypted sessions, and whether the server appears to be a dangerous open relay. An open-relay result is serious — it lets anyone route mail through your server and quickly lands you on blacklists. A healthy server advertises STARTTLS and refuses to relay for unauthenticated senders. The check runs from our network with private and internal addresses blocked.
Frequently asked questions
What is an open relay and why is it bad?
An open relay forwards mail for senders who are not authorized to use it, which spammers exploit and which gets your IP blacklisted fast. A correctly configured server rejects relay attempts from unauthenticated clients.
Why does STARTTLS matter?
STARTTLS upgrades an SMTP session to an encrypted connection, protecting message contents and credentials in transit. A server that does not offer it sends mail in plaintext.
What does the SMTP banner tell me?
The greeting line identifies the mail software and hostname. It confirms the server is answering and can reveal version details worth keeping current.
Can you alert me if my mail server goes down?
Yes. SJ Monitor offers port monitoring that checks the SMTP service continuously and notifies you the moment it stops responding.
Can I test any mail server?
Only test servers you own or are authorized to. Private and internal addresses are blocked, and the tool is rate-limited to prevent abuse.
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