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.

##</code><code>##

uniquename.dyndns.org. IN A 222.111.222 (dynamic ip)