The mailhop.org DNS is handled by DYN, we have now added Amazon’s Route 53 and Cloudflare as redundant DNS providers and have created multiple zones for redundancy. Not only are these domains registered with difference registrars, they are also utilizing independent TLD’s (Top Level Domains) and different DNS providers.
To you should configure your DNS so that your custom MX record or mx1.mailhop.org is set to priority 10 and the additional records should be configured like this:
Email Gateway / Email Forwarding
MX 10 Your custom MX Record (from customer portal) or mx1.mailhop.org
MX 20 mx1.mailhop.us
MX 30 mx1.mailhop.co
MX 40 mx1.mailhop.co.uk
MX 10 Your primary email server
MX 20 Your custom MX Record (from customer portal) or mx2.mailhop.org
MX 30 mx2.mailhop.us
MX 40 mx2.mailhop.co
MX 50 mx2.mailhop.co.uk
We are urging customers to make this change as soon as possible to minimize any future DNS related issue.