Labels with special characters (periods, @, etc) can be assigned to bookmarks (either when first adding a bookmark or when editing the bookmark) without Confluence throwing up an error like it does when you try to add them to a page or if you use the Label Edit link (tested in 2.7) .

However if you Browse Space > Labels the labels with a period in them are displayed but when you click on one of them it says "The page you were trying to reach does not exist. You may want to try a search, or browse the site to find the page you were looking for."

Special characters should not be allowed to be added to bookmarks until they are fully supported, esp. since there is no way currently to do a global rename of a label should a lot of bookmarks be added with labels containing special characters.