Are you using ACT! Email with and Internet Mail account? If so, and you are unable to delete this message from the Outbox, then you may need to contact ACT! Technical Support to use an email utility to try and delete the message. You can reach Support by calling (800)927-3989, or of you have a support contract, call them on number provided with the plan. Support hours are Monday-Friday from 8:30am to 8:30pm EST.
Note: Effective 6/1/13, Sage no longers provides support for the Act! software. This is now provided by Swiftpage.