Wpfroms does not send email even with SMTP

Wpforms is not sending notifications. I have configures WP Mail SMTP with an email account on my work domain. The test email gets delivered fine.

If I do a “Lost Password” request, the password-reset email is also delivered. On the test environment emails were delivering fine though for wpforms.

The test environment has PHP 7.1 and web admin is via cPanel
The live envirnment (on which emails are not delivering) is on PHP 5.6 and web admin is via ISPConfig

I’m not sure where I should start diagnosing the problem or what questions to ask the host for the live environment.