How can I diagnose if the 421 error is due to a firewall blocking the SMTP connection?

Question

Grade: Education Subject: Support

Asked by: USER1816

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Answer (86)

USER2142

Best Answer

(318)

To check for firewall interference, temporarily disable your firewall (or create a rule to allow outgoing connections on port 25, 465, or 587 – the common SMTP ports) and attempt to send an email. If the connection succeeds, your firewall is the culprit. Re-enable the firewall and configure it to permit SMTP traffic.

Answered on 28 Agustus 2025

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How can I diagnose if the 421 error is due to a firewall blocking the SMTP connection?

Question

Answer (86)

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