I just saw that sparkpost even encourages discourse admins to use their service:
So I wonder if it might make sense for the discourse team to contact them and ask them if they can do something about the problem of banned accounts. @zogstrip actually made a good point when he suggested that part of the unpleasant experience may be due to the suspension being automated:
I can imagine that they have piles of automated suspensions which are legitimate so that responding to complaints about suspensions on their free plan may not be a priority.
But if they (or someone) at sparkpost is willing to work with us here on meta to improve their detection algorithm, that could help solve the problem. For example, if there’d be a way for sparkpost to recognize that mails are being sent from a legitimate discourse instance, they could use less restrictive filters. Or if they could tell PM emails (which are sent to only one or a few people) these could be treated differently.