If your website is www.yourdomain.com and you are not able to send an email from a form to firstname.lastname@example.org.
This could be a possible fix if you are using cPanel.
Change your remote routing in cPanel to “Remote Mail Exchanger” this might fix the problem.
Location of Email Routing in cPanel:
Change Email routing to “Remote Mail Exchanger”
Emails from your domain name should now be sent and received without problems.