Frog is a great idea.
However I think for bitmessage to be a success there needs to evolve a market for pow, people are not going to want to drain mobile computing resources to send potentially 100s of benign msgs in 1 day. Or trade pow credits from other computing resources
Just my opinion but I think that is an important missing piece
You read my mind.
I already have a "stamps" program planned. For about .3 mBTC, you will be able to purchase a credit (or "stamp") that gets your messages processed by powerful PoW servers will a lot of GPUs.
This is how the service will pay for itself, and be usable on mobile devices.
you may want to think of monthly/qtrly/annual flat fee instead, maybe stage it for personal, business, enterprise, etc - think of it like a mailchimp service or hosted email. or for the phone app just give it away for free trial then some low yearly cost for up to XXXX msgs
maybe for large volumes you can do the per mail tx but otherwise you will just be processing for such small amounts and it will eat up cycles, and clog up the network with these micro tx.
to promote it let people send a couple hundred msgs for free per month if they had an account on here since some day (like what ripple/opencoin did) for the first year or whatever
instead of having to invest in the hardware if you allow the community to provide this proof of work, then you can act as an intermediary, collect your fee for the service, and pay the community for providing the hardware and hosting, rather then taking that on as an additional operation. but i am unsure how capable the current system is to support something like that. this also provides incentive for the greater btc community to support using this