Re: Fixed IP?
However, if your concern is more about, for example, having your own IMAP store so you can find any messages you want, then you could add Fetchmail to the mix, which will grab messages from external mail accounts and feed them into your server.
This causes difficulty for spam processing; if your Internet-facing MTA doesn't do the spam processing you want - and if you don't own it, it probably doesn't - then you have a real problem, because the mail has already been accepted.
At that point, you can either take it on the chin, or try to do some sort of local-processing, redirecting spammy-looking mails into a spam area. But that's actually the worst of all worlds, because there is a good chane of you missing a mail if you get a flaso positive, and you still have to trawl through that spam...
For my money, there should only ever be one port of call for inbound mail. It's trivial to sort that with dynamic DNS clients. You can smarthost on the outbound mail, but inbound should have as few hops as possible.
 Smarthosting loses quite a lot of the traceability of your email, so debugging a problem can be tricky. But if you've got a rDNS entry that looks like a residential account, you're bot going to have much choice.