I think SME only supports one dynamic dns entry.
Feel free to use "cname" entries for any number of domains (for browsing), or setup the MX records for your sub-domains to use the single dynamic dns entry for email..
myserver.dyndns.org ==> sme primary domain
www.mycustomer1.com in cname myserver.dyndns.org
mycustomer1.com in mx 10 myserver.dyndns.org
mycustomer2.org in mx 10 myserver.dyndns.org
Avoid "branding" complaints from your customers by making "myserver.dyndns.org" as generic as possible.