We came across this same problem with a client that was using the same Domain name for office365 and their website. When the client would use their own email address within the contact form the email would not get delivered, but when another email address was used it worked fine.
We decided to use the Microsoft email, every Office 365 email address has a corresponding Microsoft email address.
For Example if your company Name is ‘ACME Corp’ and your domain name is ‘ACME.com’, your email address is [email protected] and your corresponding Microsoft email address would be [email protected].
The Microsoft email address typically uses the company name originally setup.
Once we made this change the contact form worked perfectly. Check your Microsoft Office365 portal to verify your Microsoft email address.