What does “invalid recipient address” mean?

Invalid recipient address covers several validation failures:

Possible meanings:

User does not exist: No mailbox at this address. Format invalid: Address syntax is wrong. Validation failed: Server-side checks rejected the address. Policy restriction: This address type not allowed.

Format issues to check:

Missing @ symbol. Invalid characters. Domain portion issues. Local part too long.

Typical codes:

550 with "invalid recipient." 553 with "mailbox name invalid." 5.1.1 (bad destination mailbox).

Response:

Verify address spelling and format. Suppress if confirmed invalid. Check for typos in collection process.

Invalid recipient is a malformed destination label. The address cannot be processed as written.