Understanding cPanel SMTP Restrictions: Security, External SMTP, and the Trade-Offs

Short summary

cPanel's SMTP restriction setting blocks non-root users from connecting directly to external email servers like Microsoft 365, improving security but potentially breaking legitimate applications. The post explains the setting's purpose, common failure scenarios, and provides troubleshooting techniques to diagnose whether restrictions or network issues are causing SMTP connection errors.

• •SMTP restrictions route mail through local Exim server, preventing direct external SMTP connections by non-root users
• •Legitimate applications using external services (Microsoft 365, Gmail) may fail with cryptic TLS certificate errors when restrictions are enabled
• •Troubleshooting methods include testing connectivity as different users and examining TLS certificate identity mismatches

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