lammert - 10:54 pm on Nov 19, 2010 (gmt 0)
You might want to check the MX record of the problem domain. Often small businesses use a third party email provider to process their mail. The MX record will in that case often give information about which third party email provider is used.
For example if mail for example.com is processed by the server smtp.someisp.com, you might be better of contacting the technical support of someisp.com than sending mail to postmaster or abuse. Those aliases are often catched by the domain owner, not by the technial support people.