Member Subscription Signup Issues with Specific Domains

I have Ghost installed (and updated) on a Digital Ocean droplet, with Mailgun configured as per the guide documents, and running fine (emails go out to the subscribers like clockwork).

Completely by chance today I discovered that trying to subscribe to the blog via the “Subscribe” button in the top of the Casper (v.3.0.5, stock) theme that the modal subscribe form wasn’t working - but only for specific domains.

It seems the member sign-up function reports “Please enter a valid email address!” for email addresses. I tried a variety of different common domains (yahoo, gmail, etc) - and they seem to work - only outlook does not.

Any ideas what’s going on ? I’ve searched other community posts, and there are a few similar sounding reports, but nothing exactly the same.

I found out what is causing it - mailgun failed to send to because the IP address of the Digital Ocean droplet was listed by spamhaus - which in turn had picked up the IP address from CBL. I imagine a machine had existed in the past on the IP address at Digital Ocean, used to send spam.

I was able to manually remove the block from CBL, so now I’m waiting for Spamhaus to update.

Just a note to the devs - the message coming back from the email submission form was misleading - perhaps something about failure to send the confirmation to the destination server might be more helpful ?