Colo box (I'll toss the various virtual machine and chroot jail hosting
solutions into that).
Some flavor of VPN account where you can keep a nice static IP address
for your mail server with proper forward and reverse DNS.
Business class account with your ISP.
Some other 3rd party mail provider.
Warning... even if you secure your email, the idiots on the other end
won't.
I deal with lawyers that still insist on POP3 in the clear for their
crack berry to retrieve email.
I deal with lawyers and accountants that think a boilerplate disclaimer
will prevent someone from forwarding on a mis-directed email.
