It's making life difficult, to say the least.
We streamlined a lot of our correspondence by using HTML in e-mail...now it is text-only and we've gone back to including attachments instead of links.
The load on our Outlook Server has gone way up - many of the documents we attach are 2-5 Mbytes in size and go to multiple recipients.
And those damn sigs crash your Outlook if you go to fast :)
Don't forget that one :)
You could enclose your corporate inside a VPN with a webserver if you wished to do secure HTML mail.
However, companies wishing reasonable security have long ago - since the first major worm infection via HTML - banned HTML email.