One of the things that's been annoying some of our users is the contact selection menu. Quite often, we have to forward a mail (with the exact same formatting, subject and attachments, so composing a new mail won't do), when you click on forward, the only way to set the recipients of the message is either by typing on the window and using auto-completion or by using the "select addresses" menu, and this last one is the one I'm receiving complaints about.
It should be resizable - When you are working with large contact lists, the ability to see them all listed together greatly speeds up the workflow, but this menu is an embedded window inside the compose mail, and it cannot be resized. Therefore, some lucky people (like my boss, that is ;-) ) who have these large 22' screens, and are expecting to see a lot of information on them, are instead confined to the small window, designed for 800x600 screens. This is even worse a problem when you have many contacts whose email address begins the same (like firstname.lastname@example.org), because of the small window, the visible part is always the same, and you have to hover your mouse over each one to get to see the meaningful part of the address.
You should be able to add groups of people at once - Though not quite as important as the resizing issue before, it would be nice if you could add all recipients in a group (all in a contact list, or maybe tagging them if it were possible..), a bit like a distribution alias would work, but user-modifiable, and without having to ask the administrator to create a new alias each time this is needed.
Thanks for your attention, and sorry for the long post.
And keep up the good work!