*"...tightly secure ISP server"

I have yet to encounter a "tightly secure" ISP mail server.

Or do you mean domain host?

I have unlimited e-mail accounts through my domain host and can configure each box to accept e-mail from those I specify; everything else is flat out rejected and sent back as user unknown.  Most decent domain hosts offer this type of configuration.  An ISP, forget it.  Verizon is my ISP and I believe I'm allowed up to five e-mail accounts.  I can set one up and within a week I'll get spam.  Because of this, I've abandoned any thought of using my ISP's (Verizon) mail server.  The only thing I use Verizon's mail servers for is sending e-mail through my domain host, not to receive.

For Renderosity, I use a Yahoo account which I don't care about.  I never use it at all.  As a matter of fact, I can't even remember the password for it.

If I were you (if you're not already), I'd try a domain host.  Buy yourself a domain; domain hosting is dirt cheap these days.  Just make certain the web interface is decent and the e-mail configuration is of high-quality with decent features.  And go with a well-known host.

Regards.