I'm surprised you are not having problems by people just
blocking email from you since it is from a dynamic-IP address,
regardless of what is in the DNS.

Note that some email servers want SMTP-authentication. However,
there is another solution, as implemented by my dynamic-DNS provider:
accept email from current active customers (he knows my current IP
address already, of course). There is also no charge per email; it's
a flat rate. (However, if someone sends more than a "normal" amount
of email, transmission will be blocked so that the problem can be
sorted out. The provider wants to avoid his server from becoming
blacklisted. If one has legitimate mass emails (e.g. if you are
running a list server or something), then one can register these
special addresses. So, no hassle, everything works, costs are minimal
and if everyone did this there might still be a lot of spam, but
no-one would see it.

Email me privately if you have any experience with or thoughts
on tarpits. (For obvious reasons, discussion of this topic might
not be good to have in public.)