I don’t have much to add to Al’s post about the lack of people to call at different ISPs to get mail delivered. I will say there was a time some ISPs had staff that would deal with senders and blocking problems. But those positions have gradually been eliminated over the last 2 or 3 years. In some cases the employees left for greener pastures, in others they were subject to layoffs and budget cuts. In most cases, though, the employees were not replaced.
ISPs have moved to complex and multi-tired spam filtering. They’ve removed the ability of most employees to actually interrupt the filtering and special case a sender. Getting mail delivered is about sending mail that recipients want. It’s not about who you know. It’s about how much recipients like your mail.