How to use SMTP module with custom PHP snippet (simply using mail function, workflow module executes it)

I have a script that executes based on a condition set by the workflow-ng module, and it basically just creates an email.

I'm using the mail function, like so: mail($to, $subject, $body)

SMTP module is enabled and working, it works for the rest of the site. But when it executes this custom PHP snippet, it still uses the mail function and sends from "nobody". How can I use SMTP module in my custom php snippet?