Symptoms
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Unable to send and forward emails from Plesk server to external servers either completely or intermittently, causing mail queue to fill up.
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Emails cannot be sent by using PHP script.
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Emails between domains served by Plesk can be sent.
Cause
Port 25 is blocked for outbound connections.
Sometimes internet service providers (ISP) can restrict or limit outbound/inbound connections on port 25 due to security reasons. Also, the Google Cloud platform has such port blocked by default, and services like Google Workspace (former G Suite) should be used to send mail.
Resolution
Note: Networking tasks and the local firewall configuration are the activities that should be handled by the Plesk server administrator, as Plesk is a part of the server infrastructure and relies on local network settings.
Note: Once port 25 is opened by your hosting/cloud provider and mail starts to work, you may safely close the message about TCP port 25 is blocked in Plesk at Tools & Settings > Mail Server Settings.
Note: For an example of mail relay configuration, refer to our official Udemy course.
Here are the general troubleshooting steps:
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Make sure that the local firewall does not cause the issue. Stop the service temporarily and try to send mail or open mail port under Plesk > Tools & Settings > Firewall;
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If the issue still persists, it means that the cause is outside of the Plesk server. In that case, contact the network administrator and discuss if port 25 can be added to the allowed ones or not.
If any of the Cloud Services are used, it is needed to check the firewall on the Cloud Service’s side.