Server-side host-based filter (e.g., blacklists): reject at SMTP Server-side content-based filter (e.g., spamassassin): usually marks up the email based on rul

Server-side host-based filter (e.g., blacklists): reject at SMTP Server-side content-based filter (e.g., spamassassin): usually marks up the email based on rules, and may either reject/discard it, or leave it for the email client Email-client filter (e.g., in thunderbird, outlook extensions, etc.): silently discards