Outlook OAuth token expired, or Gmail App Password stopped working
Outlook / Microsoft 365 connect with OAuth (Sign in with Microsoft); Gmail and other providers connect with an App Password over IMAP/SMTP. If the connection stops working: 1) Outlook / Microsoft 365 — OAuth tokens expire periodically and need to be refreshed; if automatic refresh fails, go to Mailboxes, click the affected mailbox, and Sign in with Microsoft again. Tokens may be revoked if your Microsoft admin changed security policies or you changed your password. 2) Gmail and other App Password providers — the App Password may have been revoked (often after a password change, or if it hasn't been used in 6 months). Generate a fresh one (for Gmail, at myaccount.google.com/apppasswords with 2-step verification on), then click Update credentials and paste it in. 3) Check your email provider's third-party/app-access settings to make sure access is still allowed. After reconnecting, warmup and campaigns resume automatically.