I have a similar setup: I run my own mail server at home, so email
arrives directly on my computer (actually, a cluster of computers).
In fact, I "feel" an email arriving a few seconds before the broadcast
informing me of new mail because of the characteristic sounds made by
the hard disks.

When I first started out, I noticed that I got some bounces since I
was sending from a "dialup IP address" (it is also dynamic and changes
once a day or so---no problem with short TTL DNS records). The
solution was to send email through an SMTP relay server provided by my
ISP. The cost is minimal. (My dynamic-DNS provider
http://www.dynaccess.com doesn't charge for individual services,
but rather has packages with more and more features. Even if such
a package offers one something one wouldn't use, the rates are still
good compared to other providers and some services are offered only
by this provider, as far as I know.)

At first, I got little spam, due mainly to avoiding having the new
email addresses in the clear on usenet or the web. After a while,
however, I started getting spam. (Much of it was dictionary-attack
spam; there is little that one can do to avoid that, except making
the email address as long as possible, which has some disadvantages).
I set up some anti-spam filtering, such as rejecting email from
senders, IP addresses etc which had spammed me in the past, as well
as dropping the connection immediately if email was sent to a
non-existent user. (This actually had the biggest effect, which ties
in with the suspicion that much of the spam was dictionary-attack
spam.)

A few weeks ago, spam picked up again. It seems that most of this
comes from virus-infested PCs. I suppose there are viruses which
scan PC hard disks for email addresses and use them as spam targets.
There is little one can do to avoid this (unfortunately, hoping
that folks move from PCs to better computers is probably unrealistic).
(It is interesting to see what kind of traffic gets as far as my
router. I drop some connections at the router since these are to
ports used by protocols I don't even run and presumably are the
results of viruses as well. Since no application is listening on
the port, no harm is done on my side, but I might as well drop the
connection at the router to save resources. This might make an
intelligent virus decide to move on rather than trying the IP
address again (possibly for something different).)

I am thus considering, as a next step, blocking email from
dialup IP addresses. Since sending email through a trusted
SMTP relay server is a small cost compared to all the other
computer-related costs one has, I doubt that much real email is
sent directly from dynmic IP addresses and even if someone gets
blocked, I think that my reasoning would convince him to send his
email through a trusted server, especially since I'm probably not
the only one blocking.

Note that I would not bother with reverse DNS at all. As you point
out, there are legitimate reasons for one IP address having several
CNAMEs or even A-records, though at most one of these will be
the result of the reverse-DNS lookup on the IP addresses. Also, I
would block stuff sent (directly) from dialup IP addresses.

Do you think that much legitimate email is being sent directly
from dialup IP addresses?