Joe to search for fresh options after Cousins departs.
Mark's departure means that Sam Walker is the only senior goalkeeper at the club, with James Bransgrove set to be involved in the U21s fixtures this season.
"However, he is a player who is at the stage of his career that he needs to become a number one goalkeeper somewhere and he has a great chance now at Dagenham."
"I have worked with him at every age group and he has been very loyal to the club through the years since arriving as a teenager.
"An opportunity presented itself to become a number one at another League club and although Mark was contracted to us for another season, I felt that he deserved that opportunity and wouldn't stand in his way to progress.
And Joe, who has been with Mark for much of his progression through the ranks, told the official website: "I obviously wish Mark the best of luck for the future.
Mark has been with the club for over a decade, but will start the 2014/15 season with Dagenham & Redbridge after opting to sign a two year contract with the League Two club.
Colchester United boss Joe Dunne believes the opportunity for Mark Cousins to become a number one shot stopper meant that it was a move the long serving goalkeeper couldn't turn down.