Does this mean our email is “down”?
Not always. Many 4xx errors are temporary and resolve automatically. 5xx errors usually require a correction before retrying.
Use this guide when email bounces or delays. Each code has a dedicated page with a plain-language explanation, what it usually means for your office, and the best next step.
Service not available (system problem)
Connection timed out / network issue
Temporary mail problem
Mailbox unavailable (temporary)
Message temporarily deferred
Temporary server failure
Mailbox full (temporary quota)
Command unrecognized / syntax error
Syntax error in parameters
Bad sequence of commands
Command parameter not implemented
Authentication required
Authentication credentials invalid
Invalid recipient address
Recipient address not found
User not local / please try another path
Mailbox full (quota exceeded)
Bad destination mailbox address
Sender address rejected
Mailbox disabled / not accepting mail
No answer from host (routing)
Routing loop / too many hops
Message rejected by mail policy
Access denied / policy rejection
Unauthenticated email rejected
Transaction failed (policy/content)
Rejected for security reasons
Not always. Many 4xx errors are temporary and resolve automatically. 5xx errors usually require a correction before retrying.
Sender, recipient, date/time (timezone), and the full bounce/error text (or screenshot). That’s usually enough to diagnose quickly.