Your theory would be a possible explanation if the address in question were the "from" part of the message, but as I read it the OP's alias would have to be the local part of the address and the domain would have to be for his mail server. Your scenario would send the spam to dyndns.com, not to the OP, if used for the "to" part of the envelope.
I suspect it's much more likely for DynDNS to have sold at least part of their list to a third party. Another possibility is a sniffer somewhere along the way, harvesting addresses at random... -- Jim Kyle
·
·
·