What is Google trying to tell me?

I receive on a regular basis emails from [email protected] entitled

Report domain: domainname.com Submitter: google.com Report-ID: one long number

with one zip file attached with one .xml file inside with the same name.

I don't know if it's significant but they only seem to be for my webhosted domains.

I have no idea what I supposed to do with this info, or why/when it is triggered