Usually a credential mismatch. Re-export the mailbox from Winnr (Email Users → Export) and compare the username, password, host, and ports with what's in your sending tool. Winnr mailboxes use IMAP on port 993 and SMTP on port 465 (SSL), with the host shown in your export. If everything matches and every mailbox on your account fails at once, check your billing: when an account is past due for several days or placed on hold, its domains are disabled and logins stop working.
Diagnostic steps
1. Re-export credentials from Winnr.
Email Users → Export → choose your sending tool's format → Download CSV. Compare against what's in the sending tool. Any difference is your answer.
2. Verify the host and ports.
Use the IMAP and SMTP host exactly as it appears in your export (it isn't the same for every mailbox, so don't copy it from another mailbox). IMAP uses port 993 with SSL, and SMTP uses port 465 with SSL. The username is the full email address. If your sending tool has custom overrides, remove them.
3. Check the mailbox exists and is ready.
On the Email Users page, find the mailbox. The status icon should be a green check. A yellow clock means it's still being set up, and a red alert means it's disabled or has an error. If you can't find the mailbox, it may have been deleted, so remove it from your sending tool too. Repeated failed logins to deleted mailboxes can cause temporary connection errors for your other mailboxes in the same tool.
4. Set a new password.
If the credentials genuinely look right but auth still fails, set a new password. On the Email Users page, click the pencil (Edit) icon on the mailbox's row, enter a New Password (at least 8 characters), and save. Then export again and update your sending tool. If warming is on for the mailbox, it gets the new password automatically.
5. Check your account status.
If every mailbox fails at once, look for a Past Due or Account Under Review screen when you log in to Winnr. Resolving it re-enables your domains.
6. Test with a mail client.
Set up the mailbox in Apple Mail, Thunderbird, or another mail client using the exported settings. If the mail client logs in but your sending tool doesn't, the issue is on the sending tool's side.
What "auth failed" usually isn't
- Not a warming issue. Warming doesn't affect whether a mailbox can log in. (It works the other way round: warming needs the mailbox's login too. If the Warming page shows Problem for a mailbox, use Reconnect in that row's "..." menu.)
- Not a DNS issue. DNS problems show up as bounces or spam placement at the recipient's side, not as login failures in your sending tool.
What's next
Frequently asked questions
What causes "auth failed" specifically?
The mail server saw the login but rejected it. Usually that's a wrong password, a wrong username (it must be the full email address), or the wrong host or port. It can also mean the mailbox or its domain is disabled.
I just created the mailbox. Why can't my tool connect?
Mailboxes are created in the background. Wait until the mailbox's status icon on the Email Users page is a green check (a yellow clock means it's still being set up), then export and connect.
I changed the password. How do I update my sending tool?
Export the mailbox again from Winnr (Email Users → Export) and update the password in your sending tool. If the tool doesn't handle credential changes well, remove the mailbox from the tool and add it again.
I changed the password and now warming shows Problem.
Winnr normally passes a new password to warming automatically. If the Warming page still shows Problem for the mailbox, open the "..." menu on its row there and click Reconnect. That sends the current password to warming and restarts it without losing its history.
Every mailbox on my account stopped working at once.
Check for a billing problem first. If your account has been past due for several days, Winnr disables your domains and their mailboxes stop accepting logins until a payment succeeds. An account under review (account hold) is paused in the same way. See [Past due](/help/billing/past-due.html) and [Account hold](/help/billing/account-hold.html).
Some mailboxes work, others don't, same domain.
That's a mailbox-level issue, not a domain one. Re-export just the failing mailboxes and compare their credentials with what's in your tool.
Auth works but sending fails. Now what?
Different problem. Check the sending tool's send log for the error the receiving server returned. See [Bounces and rejections](/help/troubleshooting/bounces-and-rejections.html).