Admin can edit mail templates, with a few rules. There are two sections in each mail template: the editable portion and the uneditable portion. This allows you to customize your messaging but not change the basic functionality of the e-mail. These two sections are clearly delineated, see the example below.
Some basic html code can be applied for the editable portions of the mail templates, note the Tags Supported list at the bottom of the mail template.
Best practice is to make your edits and then send a draft to yourself or other farm admin first!