Fix onmicrosoft domains

[Divide-By-0]

Problem

Currently, they allow domains to set their own sub-domain on onmicrosoft.com. Here are the two formats:

From Domain:
voiceboxagency.co.uk
d= Domain:
wolfestone.onmicrosoft.com
Selector:
selector1-voiceboxagency-co-uk

From Domain:
certuspest.com
d= Domain:
certuspest.onmicrosoft.com
Selector:
selector1-certuspest-onmicrosoft-com

However, this means that when gmail or outlook receives an email from voiceboxagency.co.uk, they cannot validate that the signature from wolfestone in fact corresponds to voiceboxagency.co.uk. This is because the d= domain can be faked i.e. I can register my own random onmicrosoft domain, then fake the from address.

Proposed Solution

Edit the ic-dns-oracle to validate some DKIM signature along with a DNS fetch from an onmicrosoft.com domain in order to set the signed from domain url to work for that onmicrosoft extension.