I'm an angel investor here in Boston, though I'm unlikely to be a good prospect for you for a couple of reasons - I'm pretty tapped out right now, and I tend to invest locally. But I can give advice, and I'm something of an email expert, having built a long career there, including specific work in email security.
The bottom line of what might be a long discussion is that selling security is very hard, and in the SME environment, nearly impossible. The level of security provided by TLS at the SMTP level is considered adequate by most organizations, the difficulty and expense of running PKI precludes most certificate-based systems (SMIME and PGP), and the asymmetrical solutions like Zix work very poorly in a B2B environment.
I don't know what you've come up with, and am happy to hear if you've had a breakthrough that doesn't fall into one of those technical buckets, but I'm skeptical at the outset. And even if you have a solution, most SMEs are not aware they have a problem (and maybe they don't!), so selling to them, hard in the best circumstances (they buy through the channel, primarily) becomes even harder.
Happy to lear more if I haven't discouraged you enough already.