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I have a question about this particular error too - if we're getting this message because the volume is high and Marketo keeps trying them, won't we continue to get this message because of the volume? (it's like a circle!) How else can I stagger the delivery to ensure we aren't hitting a server with 400 messages all at the same time and then repeating that 400 messages over and over. I'm using a basic email campaign to a smart list of users and I would hate to have to separate groups out by domain.