Sending emails (e.g. invoices) via an Office 365 mailbox configured as the mail sender fails with a 535 5.7.139 'Authentication unsuccessful' error from Outlook/Exchange Online, even though the same mail-sender setup works fine for a different company/mailbox.

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Answer

This is typically caused by the O365 mailbox account having Multi-Factor Authentication (2FA/MFA) enabled, which is incompatible with the basic SMTP authentication IFS's mail sender uses — disabling 2FA on that specific mailbox has resolved this exact 535 5.7.139 error. Rather than disabling 2FA account-wide as a security tradeoff, check whether Microsoft 365 offers an app-specific password or a dedicated service-account exemption for this mailbox instead, which achieves the same SMTP-compatible authentication without weakening the account's overall MFA protection.

Documentation-based answer. Confirm the product and release before applying it. This answer has not been verified in an IFS environment.

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