SMTP 550 5.1.1 means the receiving system does not recognize or accept the recipient address. Confirm spelling, accepted domain, mailbox/alias existence and directory synchronization before retrying or changing DNS.
Step-by-step checks
Verify the full recipient address, including hidden whitespace or autocomplete history, and reproduce with a freshly typed address.
Confirm the recipient domain is accepted by the destination mail system and that the mailbox, shared mailbox, group or alias exists and is not soft-deleted.
For hybrid or synchronized environments, verify the object and proxy addresses have synchronized to the transport directory used by the receiving service.
Use the NDR diagnostic information and message trace to determine which server issued 5.1.1 and whether the rejection occurred internally or at the external destination.
Useful verification commands
Use commands only on systems you administer and capture the read-only output before making a configuration change.
The destination directory must resolve the intended address and a new test message must be accepted without using a stale autocomplete entry or unintended catch-all behavior.
Evidence to keep
Keep recipient address, NDR diagnostic server, accepted-domain state, recipient/proxy addresses, synchronization status and message trace result.
Frequently asked question
Can an address work for some senders and still return 550 5.1.1 for others?
Yes. Different routing paths, directory synchronization scopes or stale cached/autocomplete addresses can make the observed behavior sender-dependent.