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.
I call them on the telephone. Sometimes you have to call a few times to get someone willing to transfer you to 'the email guy' or IT in general.
Too many times the postmaster account and the abuse account DOES NOT exist on the mailbox server. The postmaster account and the abuse account are required by RFC [faqs.org]. By default they are not implemented by MS Exchange!
True, the admin and postmaster accounts should be present and directed to the right person or department, but there is no law in any country which enforces the Internet RFCs. Reverse DNS pointers are also required for email servers according to the standards, but if that rule is enforced, a significant part of all legitimate email wouldn't come through anymore.