If using cPanel, and you are using a website contact form, and the recipient is using MS Exchange or other external email - they won't receive the website contact form emails if the cPanel>Email Routing is set to local.

This can be a tough one to debug as other emails will be sent to other email addresses. Also the email delivery report will show that the email has been successfully delivered (becasue it was delivered locally and not delivered externally).

You will be look at spam blocking and SDF records... thinking its that... but in fact the email never left the local machine and was mailed to itself!

You will find the website form email (that didn't go to the recipient) in the Roundcube account of cPanel.