In order to receive mail on your server, you need to create MX records.
This can only be done if you have a DynDNS Pro, not the free service.
http://www.dyndns.com/services/dns/dyndns/
Most SMTP servers will reject mail sent from a dynamic IP, so sending mail will not work, unless you use a smarthost, like the SMTP from your provider.
Another point, is port 25 open, does your ISP allow you to run a mailserver on your dynamic IP?
You will have to let your server run 24/7.
You need to have a backup MX to receive your mail in case your server or the connection goes down.
Kind regards