I am using no-ip.com: wall.madduck.net. 86400 IN CNAME oerlikon.no-ip.org. I cannot add MX records to oerlikon.no-ip.org, so how am I supposed to receive mail for wall.madduck.net at a.mx.madduck.net then, or more importantly, use it as the MAIL FROM sender? Many domains these days just refuse mail from sender domains that don't have A or MX records.
I use everydns.net for a similar setup. It lets you create a zone with dynamic A record as it's root (with a client to run on your system for automatic updates) and then you can add static records to it.
Something like yourdomain.com. 300 IN A 1.2.3.4 (this updates automatically) yourdomain.com. 300 IN MX 10 whatever.com (this is static)The service is free, and very stable. (thought it supports only A/NS/CNAME/MX records for now - so no spf/ipv6/etc.)