Gnome Mail Notification Applet

Mail Notification monitors your mailboxes for new mail.  When new mail
arrives, Mail Notification alerts you by displaying an icon in the
notification area.  Highly configurable, and compatible with most mail
back-ends and mailbox typtes (mbox, MH, Evolution)

http://www.nongnu.org/mailnotify/

Requires libnotify, libgnome, and rarian, available at SlackBuilds.org