One of our customers noticed that Apple were sending a special mime type that the Watch would pick as the default when rendering an email.

I switched back to my ISPs DNS servers (Virgin Media in the UK). I noticed that almost anything I downloaded from a big company was orders of magnitude faster, mainly because Virgin Media have on network Akamai edge cache servers and you only get their addresses if you resolve via Virgin's own DNS servers.

I'm talking 1.5mb/s download vs. 45mb/s - a massive difference.

They hijack NXDOMAIN - but you can turn that off. https://my.virginmedia.com/advancederrorsearch/settings