Welcome or verification email not delivering

I used the register form to test the signup process this morning, it did work and I got a welcome email with verification link. I then deleted the user account and changed the setting to HTML email. From that point on I have not been able to get a single welcome/verification email delivered.

I have installed and activated two different SMTP email plugins, one of which tracks emails sent and has logged the welcome/verification being sent, but they are not being delivered, yet the admin email advising of a new registrations is being delivered every time.

I have tried unchecking the HTML email box, no joy. I have deactivated Register Plus Redux and tested the registration process, the standard WP welcome email is being sent and delivered no problem.

I’m getting really confused as to what could be causing this all of a sudden as it was working fine when I first tried it this morning. Does anyone have any ideas?

https://www.ads-software.com/plugins/register-plus-redux/