MS Outlook 2011 for Mac error code -18597 when sending mail

Outlook 2011 on an older Mac throws Error -18597, authentication failed, every time I send, though receiving works and the password is definitely right, it works in webmail.

Exchange account through the office. What is special about sending here?

Receiving fine while sending fails with -18597 is the classic signature of stale credentials in the Mac keychain plus Outlook 2011's habit of authenticating the two directions separately. Webmail working proves the account, so the fix is local.

1

Quit Outlook. Open Keychain Access from Applications, Utilities and search for entries matching your mail server and Exchange. Delete the ones tied to the account, they are the cached stale credentials.

2

Reopen Outlook, it prompts for the password fresh. Enter it and tick the save option so a clean keychain entry is written.

3

If the error persists, check the username format under Tools, Accounts: Exchange setups of that era often demand DOMAIN\username rather than the email address, ask whoever runs the server which form it expects.

4

Still failing after both, the identity database is suspect: quit Outlook, hold Option while launching it to open the Microsoft Database Utility and rebuild the identity. Sending is usually restored after the rebuild completes.

Longer term, gently: Outlook 2011 stopped receiving updates in 2017 and modern Exchange servers progressively drop the older authentication it speaks, so these errors return more often as the server side tightens. The mail data itself is portable whenever the upgrade day comes, the identity exports to OLM and converts onward from there.

Keychain cleanup alone fixed it, two ancient entries for a server name we stopped using last year. Sending again. Upgrade conversation noted.