Someone fills out a contact form on your site and you don’t get it? This might help.
I’m going to go with the post office analogy first to try to explain this. Let’s say you live in an apartment building on Main St. Someone comes to visit you and they want to send you a letter. They put it in the outbox there in the same building to get to you. I know, my analogy doesn’t make the most practical sense, but stay with me. The postman takes the letter and knows that it’s for you, but because it’s coming from the building where you are living, he just brings it back to your apartment. It all sounds good so far, but the problem is when your email is not hosted where your website is hosted. With the postman and mail, it would mean you don’t have your mail at your apartment, but you have it at a PO Box not on the same site where you’re living.
In email terms, your email host is not the same as your website host. For example, your email is hosted with Google Apps and your website is hosted with WP Engine. If someone visits your site and fills in your contact form and your local postman thinks you live where your PO box is, the form will stay in the same building. But your email is actually elsewhere. To fix this, you need to tell the local host MX records that your email is hosted elsewhere. See the screenshots below for tips.
In brief, you want to find where the MX records settings are for your website host. There you want to designate those to point to the MX records of your email host. So even though you’re on WP Engine or something else for a host, you want to point the MX records to Google Apps. Easy peasey, right? Yes, if you know where to look–and why.