Managing incoming messages to prevent spam, viruses, and phishing attacks can be a lot of work. That’s why large corporations usually install “email firewalls”, or hire 3rd party vendors (like “Barracuda Firewall” or “Postini”) to handle their incoming email.
Email firewalls often use a combination of Bayesian-style/adaptive filters, community reporting, blacklists, and a little bit of proprietary “magic pixie dust” to keep spam out of the company. Most of the time, when you can’t deliver email to a large company, it’s their firewall.
Firewalls tend to ask questions like, “Is this sender new to me? Why is he sending copies of the same email to a bunch of people in our company? Spammers do that stuff. Can I really trust this sender?”
Spam firewalls are usually a problem when you are sending campaigns to a big client for the first time. You are “new” to firewall and so you experience some deliverability troubles. They’ll eventually “learn” to let you through.
To solve the problem in a quicker way, you can ask the IT people in charge of the firewall to “whitelist” your IP addresses (or the IP address of your email service provider).