I was able to get it fixed by contacting Shopify and having them fix it. I reference this post to them and the fact that when email is switched it does in fact work. They were able to unblock the email from their end. Took the rep some time to do it, but they can fix it. Nothing wrong with your setup. Its Shopify.
Already responded above how to fix. You can go about it 2 ways.
2. Contact Shopify and tell them that they're blocking your email ~ firstname.lastname@example.org ~ after 15 minutes research they will unblock it. Nothing you do from your end will fix it. The issue is from their end. Only solution from your end is to use alternative email. If you don't want to contact them, just use another email such as email@example.com instead of whatever they blocked.
Last option is to use Apps. I had success with this. Search the store for Contact Us Form
We have the same issue that seemed to have appeared a few weeks ago. Everything worked fine before.
Emails from the Contact Form page, AND emails from the pop-up windows say Sent... but nothing arrives. Receving emails are working fine.
Store is: https://asweetridecalledclyde.com/