What’s the difference between MTA-level and inbox-level filtering?

MTA-level filtering occurs during the SMTP transaction, before the receiving server accepts the message. Decisions at this level result in rejection or acceptance, not folder placement.

Inbox-level filtering happens after the message is accepted. The server decides which folder to place it in: inbox, spam, categories, or custom folders.

A rejection at MTA level generates a bounce. The sender knows the message was refused. A spam folder placement at inbox level is silent. The message appears delivered from the sender's perspective.

Different signals influence each level. MTA filtering relies heavily on IP reputation and authentication. Inbox filtering adds content analysis and user engagement signals.

Diagnosing problems requires understanding which level failed. Bounces indicate MTA rejection. Successful delivery to spam indicates inbox-level filtering.

MTA filtering is the gate. Inbox filtering is the sorting room inside. Both make decisions, but at different stages with different consequences.