In this scenario I would like to allow SMTP traffic to my internet provider so that an application in my test environment is able to send notification messages. In this environment I use pfSense.

For this to work, you have to create a port forwarding rule on the LAN interface forwarding traffic to any IP with port 587.

Open the pfSense interace and go to Firewall rules:

Then create a new rule as follows:

Make sure you use the right (internal) IP address of the server which is sending the emails.