What’s the difference between 550 and 554 errors?

Both are permanent failures (5xx), but they indicate different problem types.

550 errors: Recipient-related failures

550 User unknown: Address doesn't exist

550 Mailbox unavailable: Account disabled or inaccessible

550 Recipient rejected: Policy blocking this recipient

550 Invalid recipient: Address format or domain issues

Action: Suppress the address. The problem is with the specific recipient.

554 errors: Transaction-level failures

554 Message rejected: Content blocked (spam, policy)

554 Transaction failed: General rejection

554 Denied: Policy or reputation block

554 Rejected for policy reasons: Explicit policy violation

Action: Investigate content, authentication, or reputation. The problem may affect all messages, not just this recipient.

Key distinction:

550 usually means "this address is bad"

554 usually means "this message or sender is bad"

Read the full error message for context. Servers include explanatory text that clarifies the specific issue. "554 Your IP is blocklisted" tells you something different than "554 Message contains prohibited content."

Both are permanent; don't retry the same message to the same recipient. But investigation differs based on error type.