Um, the convention, if you are suffering from issues sending mail, is to email postmaster@[domain], which should be a monitored address, per the terms of RFC 2821. Or abuse@[domain] for spam.

I certainly don't muck about trying to negotiate "customer support", because I am not a customer. I represent an organisation that is trying to send mail to a customer of theirs, and it's not the same thing at all. You'd be surprised at the number of organisations that have no provision for raising trouble-tickets for external parties, assuming they actually care. So it's best to just cut to the chase, or get an ACTUAL customer to raise it as a support issue with the approved channels.