In plenty of cases people might be flagged/shadow/full banned in this way, shrug and say oh well and never return. I've done that frankly, as yes, I've registered with blacklisted ips and such. I've also seen it and try to manually remediate if they reach out or express concern in an adjacent community. But there's not much you can do if you stay hygenic with a proxy, but your proxy has been used by unsavory folks. Board owners are compelled to deal with abuse and it's not unreasonable to strike out known problem sources, especially proxies. Or you just have a very funneled connection, often happening to international regions where someone causes a problem and an effective ip block will take out a large range, including collateral legit users. An important step is to offer a path to resolve if it's in error. If there's no message, no path to remediate and it 'just doesn't work', the chances of just losing someone spikes.
Usually this is IP and not email based though, I've never had an email get whacked for this. Now I can see something like this happening to emails from independent mail servers which often are pre-emptively, unfairly knocked and it takes tedious time and even money to get that straightened out. Obscure providers could see this issue. On the flipside sometimes there will be tech against gmail, proton. I don't sign up to such places.
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