I think the email space is a bit crowded right now across all the different parts in the chain ( webmail / mail apps, mailing list services [mailchimp,campaignmonitor], infrastructure serivces [sendgrid/sailthru] , other email operations). I'm not sure what angle you're looking at, but I'd take the competitive landscape into consideration.
Anything email related is also an immense amount of work. Programming is the easy part - maintaining high deliverability can be a FT job for several people.
The specifics of email aside...
I don't know many/any "General Products" that people like using. Usually a "general product" is something that you have to use because of critical mass ( Google, Facebook, Twitter, LinkedIn ) - and it's been made to be so vanilla and non-assuming, that it's usability and reward suffers. Think of all the cottage-industries built around customizing twitter, or how people always want an app that makes gmail work better (to their needs).
Someone mentioned the problems with sales... I'll go a step further: you can't do any sort of decent customer development on a broad general category. One group wants FeatureX, another group hates FeatureX and demands FeatureY. You try making everyone happy, so you end up making no one happy. User acceptance testing will be a nightmare. Featureset prioritization will cause you to pull out hair. By the time you finally get to market, someone comes around , says "We make Task X really easy for Group Y" and then everyone in Group Y flocks to it. And they love it. And everyone hates your product and hopes that other company grows into their industry. How many times do you see growing companies describe themselves as: "It's like _____, but for people in ___" .
Everyone wants something that seems custom or made for their style of work/pleasure. It's human nature. Everyone also hates that stuff they sell in supermarkets as "white bread". It's white and has a lot of random uses ( enough to be on every store shelf) but it looks and tastes nothing like real bread. The only people who buy it can't afford good bread.
It looks to me like you have a secondary question that is worth a deeper conversation -- the notion of how to approach branding and marketing when you have a targeted product and want to eventually expand it to a larger audience(s).