Unable to receive or send e-mails in Plesk hosted on AWS or LightSail: port 25 is blocked

Applicable to:

Unable to receive/send e-mails from Plesk hosted on Amazon Lightsail or AWS EC2 instance

The following error can be found in /var/log/maillog when sending emails to an external domain:

CONFIG_TEXT: dsn=4.4.1, status=deferred (connect to alt2.gmail-smtp-in.l.google.com[142.250.4.26]:25: Connection timed out)

Outgoing mail connections via 25 port are not possible:

# telnet mail.example2.com 25
Trying 203.0.113.2...
telnet: connect to address 203.0.113.2: Connection timed out
Trying 2001:db8:f61:a1ff:0:0:0:80... telnet: connect to address 2001:db8:f61:a1ff:0:0:0:80 Network is unreachable.
It is possible to send out an email message locally using the command:

# telnet localhost 25
Notifications are received in regards to this port:

CONFIG_TEXT: You cannot send emails from Plesk because outbound connections on TCP port 25 is blocked. Check the firewall settings or contact your hosting provider.

SMTP ports are blocked by a firewall from Amazon side or on the server.

For AWS instance, submit a request to AWS support to have port 25 unblocked in accordance with Amazon documentation.

Warning: If you have Amazon Lightsale please explicitly mention in the link above that you need to remove the port block for LightSale, otherwise AWS team will remove it for EC2 only.
If the Plesk Firewall is used, open mail port under Plesk > Tools & Settings > Firewall;
If the system or a third party firewall is active on the server, make sure that it is not blocking the connection to the 25,465,995 ports.
In case of an intermediate firewall, it is required to contact the service provider.