Hmmm. I’m seeing that users are having a hard time differentiating between direct emails and email notifications about forum content. I’ve seen several replies to personal messages in my noreply@ address inbox because they saw the entire PM content in the notification and handled it like a normal email reply. (I don’t have replies via email enabled in Discourse.) So it seems fairly likely that those users will see a spam link and without thinking twice hit the spam button. Also I’m assuming the Bayesian spam filters that the providers run will also see links to spam domains in the email body, so they might even automatically ding the sending domain/IP reputation.