To send email over the SMTP protocol using the Office 365 platform, the account being used has to be configured to allow SMTP authentication.


What to do (Microsoft 365 administrator)

  1. Open the Microsoft 365 admin center (admin.microsoft.com).
  2. Go to Users > Active users, then click the name of the account to configure.
  3. In the configuration pane, on the Mail tab, under Email apps: click Manage email apps.
  4. Tick Authenticated SMTP.
  5. Save changes.

The account setting takes precedence over the organisation setting: there is no need to enable SMTP for the whole company, and Microsoft's own advice is to enable it only on the accounts that need it.


Official documentation

Enable or disable SMTP AUTH in Exchange Online


The checkbox is missing, or ticking it has no effect

If security defaults are enabled in the tenant, Authenticated SMTP is disabled whatever this checkbox says. Those defaults have to be turned off first, in Microsoft Entra ID.


PowerShell alternative

With Exchange Online PowerShell, for one account:

Set-CASMailbox -Identity user@example.ch -SmtpClientAuthenticationDisabled $false

To check the current state:

Get-CASMailbox -Identity user@example.ch | Format-List SmtpClientAuthenticationDisabled

False = enabled, True = disabled, blank = the organisation setting applies.

Matching error message

While the setting is disabled, sending fails with an authentication error of this kind:

535 5.7.139 ... SmtpClientAuthentication is disabled for the mailbox

WinEUR cannot check this setting in advance: Microsoft does not expose it to the account itself, only to an administrator.