Hedger led the growth of e-mail archiving company KVault from its inception in 1999 to its sale to Veritas for £114m in 2004.
Mimecast's software-as-a-service e-mail management system has more than 1,000 users and the company has recorded growth of 300% year-on-year since it was founded in 2003.
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Hedger said the company's immediate goal is to launch into the US market in 2008 to complement existing UK, South African and offshore operations in the Caribbean and the Channel Islands.
"Analyst reports on software-as-a-service consistently point to e-mail as a key application to move to an outsourced web-based service and we are determined to establish Mimecast at the forefront of this market trend," he said.
Peter Bauer, chief executive at Mimecast, said Hedger's appointment was expected to be instrumental in accelerating the company's growth as a global player in web-based e-mail management.