Hello,

What I could suggest here is using an SMTP plugin for your Wordpress site.

That way you will be able to specify an SMTP server with authentication which your emails would be going through. You could, for example, use Gmail’s SMTP settings. This would drastically increase the delivery rate of your emails as well compared to using plain PHP mail.

I’ve been using this plugin for a while now and it works very well: Easy WP SMTP

Hope that this helps!
Regards,
Bobby