Gmail
How to mark all unread emails as read in Gmail
1: When logged-in to your Gmail account, in the search box up top, type the following, then press the search button: is:unread
YAHOO
How to mark all unread emails as read in Yahoo!
1: When logged-in to your Yahoo! email account, right-click on "Inbox" in the left-hand sidebar, then select "Mark All Read."Hotmail
How to mark all unread emails as read in Hotmail
1: When logged-in to Hotmail, right-click on "Inbox" in the left-hand sidebar, then select "Mark all as read."Source: http://www.zdnet.com/blog/btl/how-to-mark-all-unread-emails-as-read-in-gmail-and-more/80754