Block all outgoing traffic, but allow for mail ( port 587 )

Firmware is version : 3.0.0.4.382_50470-g033b865

I can use the parental control, but that blocks all ports.
The email port should stay open.
Reason :

I have a Dahua IP Cam, which seems to establish a lot of connections I did not ask for.
But the Cam needs to send email for the motion detection.
How can I achieve this ?