— Immediate Diagnosis Using Manual SMTP Transaction

If email is not being received even though the port is “open,”
the problem is often not the network but rejection at the SMTP policy / filtering stage.

In this situation, do not guess using a mail client.
Open an SMTP session directly and diagnose the issue at the RCPT stage.

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Connect to the SMTP Port (Check the 220 Banner)

Connect using either of the following:

nc -v <SMTP_SERVER_IP> <PORT>

or

telnet <SMTP_SERVER_IP> <PORT>

If the connection is successful, you should see a banner similar to:

220 smtp.example.com ESMTP Postfix

If 220 does not appear, this is not an SMTP policy issue.
It is a network / firewall / security group / listening service problem.

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Manual SMTP Transaction Test (RCPT TO is the Key)

If you received the 220 banner, now enter the following one line at a time, pressing Enter after each:

HELO test.com
MAIL FROM:<sender@fake-domain.com>
RCPT TO:<realuser@yourdomain.com>

• HELO test.com
  The domain can be anything.
• MAIL FROM
  Test sender address. Anything is acceptable.
• RCPT TO
  Must be the real recipient address you are testing.
  Otherwise, the test is meaningless.

When finished, close the session:

QUIT

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Result Interpretation

Response	Meaning
250 Ok	SMTP server accepts the recipient
550 sender domain not allowed	Blocked by sender domain policy
554 Transaction failed	Blocked by policy / filter (spam, security module, etc.)
421 Service not available	Server is rejecting the session (service / resource / policy issue)

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Why RCPT TO Is the Critical Step

In SMTP, the decision of accepting or rejecting mail is made primarily at the RCPT TO stage.

There is no need to proceed to the DATA stage.
If RCPT TO returns 250, the server will accept mail.
If it fails here, the problem is in policy / filtering / relay configuration.

🛠 마지막 수정일: 2025.12.24