Double opt in not working

I am not using an email sending service for now, but I have sent test emails to a few of my email accounts and the emails come through fine (tests sent from within FluentCRM). However, when I test a new subscriber sign up, the double opt in email simply NEVER gets received?

The contact email address gets added to the contacts list with ‘pending’ status. But the opt in email is never received. Fluent SMPT displays the opt in as having been sent, but it is never received. Other emails do get sent and received.

I therefore tried to turn off the double opt in through the general settings. But this makes no difference. Any new subscriber signups get added as pending, and Fluent SMTP says that an opt in email was sent (but of course this still is never received).

What on earth is going on? Also, the email editor is failing to load as well!