I can tell this project comes directly from your own pain point, one that I have also had on many occasions.
Where you have probably fallen off track with it is by not defining your target market at the beginning you've got into the trap of presuming "extra features" = better value for the customer.
If you look at what you're selling - it's a simplification of an existing technology. You aren't in the business of selling people 'more' instead it's your job to refine, remove & simplify the process of sharing mail. Therefore every feature you add, it's just getting closer back towards the way things are done now.
From my point of view you are targeting individuals on the verge of starting a team. Bloggers with an assistant, co founders with their first users, freelancers with dedicated outsourcers.
You're the gap between a hellish inbox and really complex costly ticketing software.
A few general pointers spring to mind. I feel the word 'mail' should be in the name at least.
Your landing page should make people say 'hell yes' in 1 big headline, 3 bullet points and 2 small screenshots. A real hit between the eyes. Sell on emotional benefits like "fly through your inbox" "delegate like a pro" "goodbye repetitive e-mails'.
Also you have to make your onboarding as frictionless as possible. People with 1 well managed assistant will eventually get 2 or 3. They will think about future costs before they sign up with you. You want to HELP your users expand their team, so make it clear up front that it's pretty cheap to add extra e-mail addresses.
Some flat pricing to instill trust (we won't screw you over when you expand) plus a low (paying) entry tier. e.g $5 per e-mail flat. Make it a no-brainer to start (and then stay) with you as they grow.
Don't forget, e-mail is crucial to a business - I don't know how you're fixed for offering awesome support but that would also be a big selling point for your market segment.
Take a good look at your running costs and see how many users you'd need to break even at different price points. Also how many users it would take to cover costs & your existing salary after taxes.
I could go into further detail, just shout up if needed. Good luck!