At what point is an email filtered?

You hit send. Now what? Most people imagine a single gate where spam filters either wave your email through or block it. The reality is messier. Filtering happens at several distinct points, and a message can get stopped (or silently redirected) at any of them.

Stage 1: The SMTP connection

The first checkpoint happens before your message body is even transmitted. The receiving server looks at your sending IP, checks it against Spamhaus and other blocklists, and validates your SPF record. If something looks wrong at this stage, the server issues a hard rejection (a 5xx error) and your message never gets read. No inbox, no spam folder. Gone.

Stage 2: Content filtering

But if your message passes the connection checks, the receiving server accepts it and starts scanning the actual email. Headers, body text, links, and attachments all get evaluated. This is where spam scoring algorithms weigh in. Your message might be accepted but quietly routed to the spam folder based on what's inside it.

Stage 3: Post-delivery filtering

Even landing in the inbox doesn't mean you're done. Providers like Gmail and Outlook run background scans after initial delivery. Spam reports from recipients can trigger retroactive reclassification, moving messages out of the inbox after the fact. Your open rate report might show a delivery, but the message ended up in spam anyway.

Which stage is hardest to pass? Connection filtering is the most binary (pass or fail, no negotiation), but content filtering is where most legitimate senders run into trouble. It's also the stage most affected by your sending history and authentication setup.

If you're trying to figure out where your emails are actually getting caught, our free Email Header Analyzer can help you trace what happened at each stage. Or if something's actively broken, the SOS hotline is free.