I also use dyndns, and find it to work wonderfully for my purposes (with my own domains as well).

With Tom's suggestion you will not need to receive at email@yourdomain.dyndns.org, you will still be able to accept mail at email@yourdomain.com.

The changes at your dns host would look something like this:

OLD:
www.yourdomain.com.          IN A 111.222.111 (static ip of webserver)
emailserver.yourdomain.com.  IN CNAME homecomputer.yourdomain.com.
homecomputer.yourdomain.com. IN A 222.111.222 (dynamic ip)
yourdomain.com.              IN MX 10 emailserver.yourdomain.com.

NEW:
www.yourdomain.com.          IN A 111.222.111 (static ip of webserver)
emailserver.yourdomain.com.  IN CNAME uniquename.dyndns.org.
yourdomain.com.              IN MX 10 emailserver.yourdomain.com.
[download]

In the NEW: example, your dns host no longer needs to update any ip addresses. DynDNS would have a record that looks something like this:

uniquename.dyndns.org. IN A 222.111.222 (dynamic ip)
[download]

You would run a DynDNS client (Perl script?) that automatically updates that ip address as it changes.

With dyndns.org keeping track of your routable ip address, your dns host only needs to know that they can reach that server at uniquename.dyndns.org.

This will allow you to sit back and relax, knowing that even if your ip address changes everyday, email@yourdomain.com will still be delivered to your email server without you having to call anyone.

FWIW, the last router (Netgear) I bought for my house has a DynDNS client installed on it, so I don't need to run the client on one of my computers.