Unable to send and forward emails from Plesk server to external servers either completely or intermittently, causing mail queue to fill up.
Emails cannot be sent by using PHP script.
Emails between domains served by Plesk can be sent.
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.
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:
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;
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.
|References to unlock SMTP ports
Outbound connections to port 25 are limited by default to prevent outgoing spam.
outbound connections to port 25 are limited by default to prevent outgoing spam.
Azure blocks port 25 for new users.
Submit a request by using the following link:
Subscription Management Problem type: Request to enable Port 25 email flow
|Use a third-party service like SendGrid. Create a SendGrid account and configure Postfix to use SendGrid as a relay.
|To unlock port 25 follow: https://www.vultr.com/docs/what-ports-are-blocked
|Block the ports 25, 465 and 587 by default but can unlock them (see point 3) https://server.nitrado.net/eng/pages/cloud_server_tos
|Configure firewall on port 25: https://docs.ovh.com/ca/en/dedicated/firewall-network/
Alibaba ECS instances
|Apply the form to enable TCP port 25.
Ports 25, 465 and 587 can be unlocked:
|Linodes on newly created customer accounts will have connections from ports 25, 465, and 587 blocked by default.
Contact DigitalOcean support