Legal Question in Technology Law in California

You need to meet your clients' reasonable expectations. Anything else would be a breach of contract and you could be sued. One way to control their expectations is to have carefully set-forth conditions under which messages will be deleted. This could include reserving a right to delete messages that are offensive, libelous or violate copyright. However, this right would need to be exercised wisely, as folks' views may differ. You could also charge more for long-term accumulation of messages and/or have a fixed deletion date. Just be clear in advance what the policy is, and stick to it.