Exchange 2013 SMTP relay messages disappearing when destined for local domain mailboxes

Hello!  I have an Exchange 2013 on-prem server that is relaying email from a contact us Wordpress form on the website using a domain account; when the recipients are part of the same domain as the sender, the messages don't seem to make it to the recipients .... when I add an outside email address such as my gmail address, those messages deliver.  I am searching for some assistance in reviewing the logs to figure out where and why this is being blocked in Exchange and appreciate the suggestions.  TIA!