All emails send except new user registration

Hi there,

So our plugin is configured correctly, test email works, password reset email works, form emails work.

However we cannot get the confirmation email to send out when adding a new user to a site.

WordPress says the account has been created and confirmation email sent but the email never arrives.

Any ideas? We are on the latest version of the plugin and WordPress, running it in multi-site. The plugin is configured on the root site and on all the sub-sites.