said by Snowy:
How unique is the local part of the email address?
It's not uncommon for spammers to reuse the local part of an email address list by replacing the domain part with a different name.
firstname.lastname@example.org becomes email@example.com, firstname.lastname@example.org etc...
Assumes that OP has an '@dyndns.com' email account; but I don't see any such service at their site. OTOH, I have long had a paid DNS service from them, and opted to receive regular notices from them to an old Pacbell account, using Yahoo! Mail Addressguard.
My address is in the form, 'email@example.com'. In my case, Yahoo! has had insecurities, such that, 'firstname.lastname@example.org' for any given Yahoo! Mail (or partner ISP) account might have been leaked.
In the OP's case, I assume he is familiar with the security of his ESP's servers, so it would br reasonable to consider a problem with the DynDNS subscriber mailing list.--
~Oh Lord, why have you come
~To Konnyu, with the Lion and the Drum