I'll say what I said the last time something like this came up, which was something like last week: It is impossible for you to prove that you've destroyed the data. That requires a Trent, a trusted party; it is unverifiable.
Especially in the US, Trent is one NSL away from being falsely trusted (see Lavabit). In fact, as far as I can see, you're not even positively claiming you are destroying the messages: "We may retain personal information indefinitely." Are you able to explain that item in the privacy policy?
In general, please try not to design cryptosystems which require Trent in the post-Snowden era. That is a sign you've either failed in your design, are trying to solve the wrong problem, or aren't being creative enough.
I'll say what I said the last time something like this came up, which was something like last week: It is impossible for you to prove that you've destroyed the data. That requires a Trent, a trusted party; it is unverifiable.
Especially in the US, Trent is one NSL away from being falsely trusted (see Lavabit). In fact, as far as I can see, you're not even positively claiming you are destroying the messages: "We may retain personal information indefinitely." Are you able to explain that item in the privacy policy?
In general, please try not to design cryptosystems which require Trent in the post-Snowden era. That is a sign you've either failed in your design, are trying to solve the wrong problem, or aren't being creative enough.