Almost always credential mismatch. Re-export credentials from Winnr — password may have been rotated, or the export you're using is stale. Confirm SMTP host, port, and username are correct. If credentials match and it still fails, check whether your sending tool's IP is being blocked (rare, but possible for shared sending platforms).
Diagnostic steps
1. Re-export credentials from Winnr.
Email Users → Export → your sending tool's format. Compare against what's in the sending tool. Any difference = that's your issue.
2. Verify SMTP host and port.
For Winnr mailboxes, SMTP settings are consistent across the fleet. If your sending tool has custom overrides, remove them and use Winnr's defaults.
3. Check the mailbox exists and is active.
Open Email Users, find the mailbox. Status should be "Active." If it's paused, deleted, or in an unknown state, that's why auth fails.
4. Rotate the password.
If credentials genuinely look right but auth still fails, rotate the password on the mailbox (Email Users → click the row → Rotate Password) and re-export. Sometimes a stale credential gets stuck.
5. Test with a mail client.
Set up the mailbox in Apple Mail, Thunderbird, or another mail client. If mail-client auth works but your sending tool doesn't, the issue is on the sending tool side (their IP is blocked, their auth is misconfigured, etc.).
What "auth failed" doesn't mean
- Not a Winnr subscription issue. Auth failures don't happen from billing problems.
- Not a warming issue. Warming state doesn't affect SMTP auth.
- Not a DNS issue. DNS failures manifest as bounces at recipient side, not SMTP auth at sender side.
What's next
Frequently asked questions
What causes "auth failed" specifically?
SMTP-level authentication rejection. Server saw the credentials but they don't match. Usually — wrong password, wrong username, or wrong port.
I just created the mailbox. Why can't my tool connect?
Mailbox propagation takes 5-15 seconds. If you exported credentials the same second you created the mailbox, wait 30 seconds and retry. Also — some sending tools cache DNS aggressively; may need to restart the tool.
I rotated the password. How do I update my sending tool?
Export from Winnr (Email Users → Export). Re-import into the sending tool. Delete the old mailbox from the tool first, then add the new one — some tools don't handle credential updates cleanly.
Some mailboxes work, others don't, same domain.
Individual mailbox issue, not domain-level. Re-export just the failing mailboxes. If they use different credentials than what you have, that's the issue.
Auth works but sending fails silently. Now what?
Different problem — check the sending tool's send log. Common causes — daily volume cap hit, IP block downstream, content rejection at recipient side. See [Bounces and rejections](/help/troubleshooting/bounces-and-rejections.html).