You can use /usr/sbin/sendmail on your app container to send emails or use the PLATFORM_SMTP_HOST environment variable in your SMTP configuration.
When outgoing emails are on, PLATFORM_SMTP_HOST is the address of the SMTP host that should be used. When outgoing emails are off, the variable is empty.
You can see the variable from the app container:
platform ssh "env" | grep PLATFORM_SMTP_HOST
When using PLATFORM_SMTP_HOST, send email through port 25 (often the default). Your emails are proxied through the Platform.sh SMTP host and encrypted over port 465 before being sent to the outside world.
You can test the service:
printf "From: firstname.lastname@example.org\nSubject: Test \nThis is a test message" | /usr/sbin/sendmail email@example.com
In a little while, the test message should arrive at the recipient address. Be careful to test with real email addresses.
To review the whole process of setting up Mail service, please see our Docs.