Here's my 2 cents which you can dispense with as desired:
I'm wondering why you are choosing to make "members" subsidize free users.
Why do you want free users? Because they will convert to/recruit paying customres at some point? Well if that is the case then you don't really need to charge more, other than to cover a short term cash-flow problem. However, If you don't think they're going to convert in sufficent %, then why do you want them at all?
What I would hate to see is fastmail go through the growing pains that a lot of other services have experienced when they have a great service for free and too many moochers join on and the business model never materializes.
It seems to me that if there are too many free users some better options might be:
1) Give free users much less resources/features (i.e. IMAP/POP, bandwidth, storage, etc...)
2) Make it free for 6 months only.
3) Not have free users, and maybe have a $3 guest membership. I know that the credit card company would eat a chunk of this, but it will at least qualify your customers into the kind that would pay and likely give you a much higher conversion rate.
4) Let paying users who beleive in the model cover the cash shortfall, but get rewarded later. Maybe you could charge $14 but give a $5 credit after 6 months, that could be used to buy extra storage, etc... Or if the conversion numbers are higher than expected, give a $10 credit.
5) Just threaten to increase the price to encourage conversion, but never really do it.
Just some thoughts.
- Rahul