Forbidden: Mail domain for disposable e-mails

This is the first time I’ve received this error, I have a user trying to add their email to our Campaign Monitor and we’re getting the error:

*** Forbidden. (the users email) : Mail domain for disposable e-mails. ***

Is this a new feature to try and deter disposable e-mails?
If so, could it be accidentally blocking legit emails?

That message is not coming from Gravity Forms. Where or when do you see that? Can you share a screenshot? Are you using any type of spam or security plugin that could be doing that?

Are you integrating your forms with Aweber by chance?

We’re integrated with Campaign Monitor.

Our hosting is with WP Engine and I’m pretty sure they have MU spam and security plugins installed.

Can you share a screenshot of where you are getting that message?

I don’t think it’s coming from WP Engine. We have many customers using that hosting and this is the first I’ve heard of that message.