Sometimes, you need to mail your officers to get a consensus on something. You have the officer chat, which is good if all the officers are online, but if you have officers that live in different parts of the world, it's hard to run an idea past everyone quickly.
The title kinda explains itself. Just a little button under guild mail or something, that would be cool.
As an extension for this, maybe mail to multiple people, separated by semicolons? it's a hassle mailing 15 or so different people that you need to communicate with, copying and pasting the same message.