[O365][SMTP] Thunderbird can fetch but not send emails

Question

Hey guys,

in my org we've like 6 people which are on Linux Mint (Ubuntu Jammy).

They are all using Thunderbird with O365 enterprise.

For 5 of these people its not an issue to use Thunderbird for sending emails via O365 SMTP.

Fetching emails via IMAP is working as expected.

Additional Info:

1. In AzureAD we allowed Thunderbird as Application (9e5f94bc-e8a4-4e73-b8be-63364c29d753) with all its required permissions
2. Modern Authentication is enforced
3. All users are added to this application
4. All users Thunderbirds are equaly configured
   1. IMAP settings for all user:
      Server name: outlook.office365.com
      Port: 993
      Encryption method: TLS
      Auth method: OAuth2
      oauth2.issuer: login.microsoftonline.com
      oauth2.scope: https://outlook.office.com/IMAP.AccessAsUser.All https://outlook.office.com/POP.AccessAsUser.All https://outlook.office.com/SMTP.Send offline_access
      1. SMTP Settings for all user:
         Server name: smtp.office365.com Port: 587 Encryption method: STARTTLS
         Auth method: OAuth2
         oauth2.issuer: login.microsoftonline.com
         oauth2.scope: https://outlook.office.com/IMAP.AccessAsUser.All https://outlook.office.com/POP.AccessAsUser.All https://outlook.office.com/SMTP.Send offline_access
5. We tried also installing Thunderbird in a fresh VM without anything installed but the system, it failed equally so we assume the user has to have something (not) configured in his/her account.

Logs show the following:

mailnews.smtp: Command failed: 451 Temporary server error. Please try again later. PRX5 [BE0P281CA0028.DEUP281.PROD.OUTLOOK.COM 2023-10-06T11:24:44.896Z 08DBC639945ABA63]; currentAction=_actionAUTH_XOAUTH2 SmtpClient.jsm:533:19

mailnews.smtp: Error during AUTH XOAUTH2, sending empty response SmtpClient.jsm:1058:19

Calendar: No unique email address for lookup! calItipUtils.jsm:1115

mailnews.smtp: Command failed: 500 Unrecognized command 'unknown' [BE0P281CA0028.DEUP281.PROD.OUTLOOK.COM 2023-10-06T11:24:49.911Z 08DBC639945ABA63]; currentAction=_actionAUTHComplete SmtpClient.jsm:533:19

mailnews.smtp: Authentication failed: Unrecognized command 'unknown' [BE0P281CA0028.DEUP281.PROD.OUTLOOK.COM 2023-10-06T11:24:49.911Z 08DBC639945ABA63] SmtpClient.jsm:723:17

mailnews.smtp: Authentication failed: Unrecognized command 'unknown' [BE0P281CA0028.DEUP281.PROD.OUTLOOK.COM 2023-10-06T11:24:49.911Z 08DBC639945ABA63] SmtpClient.jsm:764:19

mailnews.send: Sending failed; Unable to authenticate to Outgoing server (SMTP) smtp.office365.com. Please check the password and verify the 'Authentication method' in 'Account Settings | Outgoing server (SMTP)'., exitCode=2153066805, originalMsgURI= MessageSend.jsm:337:32

NS_ERROR_ABORT: Component returned failure code: 0x80004004 (NS_ERROR_ABORT) [nsIWindowWatcher.openWindow]

Answer 1

Hi @Danny Heinrich

Since this issue only occurs on a specific user, if you configure another mailbox on this device, would the issue persist?

If another mailbox works, please have a check in Exchange Online if SMTP AUTH is enabled for this mailbox.

In Exchange Online Powershell:

Get-CASMailbox -Identity <EmailAddress> | Format-List SmtpClientAuthenticationDisabled

If the result shows $true, please set it to $false and see if it helps with this issue:

Set-CASMailbox -Identity <EmailAddress> -SmtpClientAuthenticationDisabled $false

Besides, please make sure TLS 1.2 is enabled on the device.

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