Sites hosted on SiteGround no longer receive email notifications for contact form submissions. It may also happen with Flywheel and a few other hosts. Most hosts appear to be unaffected.

This was probably introduced by #6657 (version 4.9). Initially tracked in 385-gh-jpop-issues.

When testing the fix, it'll be important to verify it on a variety of hosts.

Steps to reproduce the issue

1. Install/configure Jetpack and WP Mail Log on a SiteGround site
2. Add a contact form to page
3. Submit the form
4. View the corresponding WP Mail Log entry and check the recipient's inbox

What I expected

The email should be received, and there shouldn't be any errors in the log.

What happened instead

The email is not received, and the log shows an error: Could not instantiate mail function.