You're correct that the mention of them bouncing as 'non-existent' is not the only way that people can be put onto that list. Any bounces of non-existent or undeliverable can adversely affect your sending reputation, so those bounces are the primary reason that we suspend accounts. They can also on occasion be suspended by request from the email address holder, or their domain, though this is relatively uncommon. Requested suspensions are also the only kind we can not remove easily for you when you contact us directly, though the process usually takes a matter of a few minutes on the phone, not several days.
We do not suspend after a single bounce, however since the suspension is a global setting, it does sometimes appear that that they were only sent once, because the other bounces were happening in other accounts.
I hope that helps understand that part of the process a little more. I do see how the suspension process could be especially problematic with the confirmed opt-in turned on, and I'm passing your ideas on to our design team, hopefully we can find a way that works well for you and other's like you, without adversely affecting your send histories and landing your other emails into spam-boxes.
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