How can spam filters reject before mailbox creation?

Spam filters can reject before mailbox verification:

SMTP processing order options:

Some servers check spam filters first. Reject known-bad senders before recipient lookup. Efficient: Why look up address for spam?

When this happens:

IP or domain blocklisted. Content triggers immediate rejection. Authentication fails upfront. Rate limits exceeded.

Implications:

Rejection may not indicate address validity. 550 could mean "blocked" not "user not found." Cannot distinguish spam block from invalid address.

Bounce interpretation:

Read full message carefully. Distinguish "user unknown" from "spam rejected." Same 5xx code may have different meanings.

Best practice:

Do not assume 550 always means invalid address. Check for spam/policy language in response.

Spam filters may slam the door before checking who lives there. Rejection reason matters more than just the code.