As simply as I can put it your mail sends through a mail server, your incoming mail uses IMAP or POP protocols and outgoing mail uses SMTP. SMTP host would be the IP address of your mail server - something like smtp.gmail.com.

Hello all,

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 SendGrid’s SMTP service. 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,
Alex