Great discussion Dan and it's very common practice for corporate SPAM systems to trip when you're sending bulk messages (i.e. webinar invites, newsletters, etc.) to multiple people within the same account / domain. I've seen some systems be as aggressive as 3 or more email contacts and then they drop the rest of them. They also don't tell the delivering system so it looks like they've been delivered. One approach is assigning a random number to each contact within an account and segmenting your lists based on the number, which can definitely help, but can be incredibly time consuming and error prone. You can also automate this - that's actually what our company does with it's send time optimization product. Reach out off-line if your interested and I can send you some more details (mike@theseventhsense.com).
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