I think the parent comment was referring to blocking access to other server instances rather than blocking individual users. Overzealous spam filters seem like

I think the parent comment was referring to blocking access to other server instances rather than blocking individual users. Overzealous spam filters seem like the email equivalent. I have reoccurring issues trying to get self hosted email received by GMail/outlook addresses. Although these filters are an impersonal corporate policy rather than up to an individual instance admin's discretion.