Looking for a better way to present a page that uses checkboxes to select from a dropdown

I inherited a web app and have been slowly bringing it up to more modern standards. This page in particular has really got me stumped. I don't know how to best present this in a better more functional way. Does anyone have any ideas that I could turn into something?

It's basically to send email messages to employees. They can select a department which in turn will select those employees in that department in the multi-select box. Or they can just pick and choose from the select box alone. The message box is CKeditor, an old version which I will update.

Here is the page. All names are fictitious.