The company installed monitoring software five months ago to identify and delete spam or e-mails with inappropriate or copyrighted content sent by the supermarket's own staff.
"It has freed up a lot of bandwidth. Most of the spam e-mails contained large graphic files," said Colin Clark, corporate cost manager for the supermarket.
"It also means we are not breaching copyright by sending or receiving music files. And there is no risk of a prosecution for paedophile pornography because there is none coming in."
The company has been able to use the filtering software, which examines keywords and checks for flesh tones, in conjunction with an e-mail archiving system to enforce its e-mail policy.
Somerfield realised that staff were failing to follow the policy when it installed the Enterprise Vault archiving system, from supplier KVS, 18 months ago.
The system has saved the company money in other ways. This month, it was able to retrieve e-mails that had been accidentally deleted when a member of staff left. The e-mails were able to resolve a dispute that saved Somerfield £45,000.