Domain registered thru Google, site hosted on Hostmonster, can’t get email to work

This may be a very niche problem, but I’m hoping someone can help. 

My domain is registered thru Google Domains. My D7 website is hosted on Hostmonster. 

I have email for the domain configured (tested and working properly) thru Google Domains. 

On my Hostmonster-hosted website, I use that email address as the site address, the contact form, etc. Everywhere I can find that requires an email address. 

But when I submit a message thru the site’s contact form, or when I try to do a “forgot password” action, I don’t receive those emails. Drupal says “message sent successfully”, and I don’t see any log issues. Just doesn’t work. 

Ive talked to both Google and Hostmonster. They both say the problem must be with the other. And they both confirm that my MX and @ records are properly configured and working. The URL properly points to Google. 

Im sure this is something simple, and I’m pretty sure it’s something I missed on the Hostmonster side. 

Does anyone have any suggestions on where I should look?

Thanks so much for any help. 

Drupal version: