You can use an email rule to print emails automatically as they arrive. First you need to open the email rule list screen. More...

Use the "+" button in the top of the rules screen to create a new rule.

Tap on the "+" button in the actions and choose the "Print" action.

You may also want to choose some options in the "Search" section such as a subject that contains some words.

This new rule will now print emails whenever AltaMail is running in the foreground.

You can view the rule overview help section for more rule editing assistance. Here