exchange transport rule relay fails with "5.7.1 client was not authenticated" on legitimate scan-to-email traffic

TL;DR

Make the scan-to-email path authenticate: point the printers at the authenticated SMTP submission endpoint with a dedicated service mailbox, or create a properly scoped inbound connector for their IPs, because an unauthenticated relay attempt will always draw 5.7.1 from a transport rule that requires auth. Prove it on one affected message before rolling it to the rest of mail on that sending or receiving path, because this fault repeats identically wherever the same condition holds and a fleet-wide or batch-wide first attempt multiplies the damage when the diagnosis is wrong.

exchange transport rule relay fails with "5.7.1 client was not authenticated" on legitimate scan-to-email traffic

Use this when

• You see this exact error or symptom: exchange transport rule relay fails with "5.7.1 client was not authenticated" on legitimate scan-to-email traffic.
• The failure is in the mail gateway, Exchange Online, Gmail or Yahoo receiving path named in the title, and it repeats on the same action rather than happening once and clearing.
• Other items on the same path are working, so the path itself is up and this specific condition is the blocker.

Not for this skill when

• A different error code, message, or tool is involved. Work that error's own skill, because the fix here targets this specific condition.
• The whole service or platform is down. Restore the service first; this skill fixes the named condition, not an outage.
• You cannot identify one affected item to test on. Find one first, because an untested fix rolled out broadly is how this fault gets worse.

Steps

1. Confirm the fault on one affected message first and capture the evidence before changing anything: check the message trace and the full Authentication-Results and rejection headers for one affected message, read on the receiving side, and record the exact error text, the record or host involved, and the time it happened. Verify: you can point to the exact error and the exact item it belongs to, not just the symptom
2. Make the scan-to-email path authenticate: point the printers at the authenticated SMTP submission endpoint with a dedicated service mailbox, or create a properly scoped inbound connector for their IPs, because an unauthenticated relay attempt will always draw 5.7.1 from a transport rule that requires auth. Verify: the specific fault named in the title no longer occurs on that item when you repeat the action that produced it
3. Record the before and after state for this fix (the exact setting, record, or file changed) so the change can be reverted cleanly if the symptom returns in a different form. Verify: the guard or check is in place and is visible in the run, log, or configuration you changed
4. Retest end to end: send or release one test message through the exact same path and confirm it delivers with the expected authentication results. Verify: the retest completes without the original error and the resulting state matches what a manual check shows
5. Roll the fix out to the rest of mail on that sending or receiving path in the same way, watching the first few for the same error, and note the root cause and the change in the run or incident record. Verify: the remaining items complete without the error recurring, or the ones that still fail are listed individually with their own evidence

Variant phrasings

5.7.1 client was not authenticated fix

The verbatim error or code phrasing for this fault in the mail gateway; the fix is the same one in the steps above.

exchange transport rule relay fails with "5.7.1 client was not

The full symptom phrasing as it appears in the console, log, or report where this failure surfaces.

how to fix exchange transport rule relay fails with "5.7.1 client

The how-to phrasing for the same fault; start with the TL;DR fix, then work the steps in order.

Compatibility: Exchange Online and Microsoft Defender, Gmail and Google Postmaster Tools, Yahoo bulk sender rules, Proofpoint, Mimecast, Barracuda and Cisco Secure Email Gateway as named in the title.

Why it happens

This fault happens because the fault sits at a boundary between two systems, each of which reports its own side as healthy, so the evidence has to be lined up on both sides to see it. In this case the condition is the one named in the title, in the mail gateway, Exchange Online, Gmail or Yahoo receiving path named in the title: the receiving or acting side reports the failure faithfully, but the underlying state (a record, a setting, a credential, a file, or a mapping) does not match what that action assumes. Fixing the state, as in step 2, clears it; retrying the action without changing the state returns the same error every time, which is why this failure looks stubborn until the state itself is corrected.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_CqSQaoIjbS5sPzj28Gk71A