Shared Mailbox Office 365

[p3root]

Hi @jstedfast !

I have some issues authenticating the SMTPClient to Office365 using a shared mailbox.
Connecting to my personal account works, but the shared mailbox is not possible.

I have attached some logs for you:

S: 220 AM0PR02CA0134.outlook.office365.com Microsoft ESMTP MAIL Service ready at Thu, 11 Jul 2024 11:46:42 +0000
C: EHLO .....
S: 250-AM0PR02CA0134.outlook.office365.com Hello [212.52.200.46]
S: 250-SIZE 157286400
S: 250-PIPELINING
S: 250-DSN
S: 250-ENHANCEDSTATUSCODES
S: 250-STARTTLS
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-CHUNKING
S: 250 SMTPUTF8
C: STARTTLS
S: 220 2.0.0 SMTP server ready
C: EHLO ....
S: 250-AM0PR02CA0134.outlook.office365.com Hello [212.52.200.46]
S: 250-SIZE 157286400
S: 250-PIPELINING
S: 250-DSN
S: 250-ENHANCEDSTATUSCODES
S: 250-AUTH LOGIN XOAUTH2
S: 250-8BITMIME
S: 250-BINARYMIME
S: 250-CHUNKING
S: 250 SMTPUTF8
C: AUTH XOAUTH2 dXNlcj1wYXRy...
S: 535 5.7.3 Authentication unsuccessful [AM0PR02CA0134.eurprd02.prod.outlook.com 2024-07-11T11:46:49.042Z 08DCA18218166FB4]

I always get the "S: 535 5.7.3 Authentication unsuccessful". I have enabled SMTP AUTH in my tenant and for my user.

Token seems to be valid, I can connect to my personal mailbox.

mailbox.Username = "office@.."; //personal user is patrik....

var token = await OAuthUtils.GetValidMailboxAccessToken(db, mailbox.Id);
var oauth = new SaslMechanismOAuth2(mailbox.Username, token); 
await client.AuthenticateAsync(oauth);

I also tried it with "patrik@...\office". Did not work either. Could you provide any help? Or do I need to ask MS.

THANKS
Patrik

[jstedfast]

You probably need to ask Microsoft Support because I don't know how to format the username string to do what you are trying to do.

[jstedfast]

If you find out, can you share the answer here? If you can do that, I'll update the FAQ to add this. I'm sure I've gotten other questions about this but I don't remember if/what the answer was.

FWIW, there were other questions about this in the past: https://github.com/jstedfast/MailKit/issues?q=is%3Aissue+is%3Aclosed+shared+mailbox

This is probably the most likely answer to your question: #1412 (comment)

[p3root]

https://learn.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth#sasl-xoauth2-authentication-for-shared-mailboxes-in-office-365

Check this link. But does not work either. I will contact MS and let you know.

[p3root]

@jstedfast
After several weeks of phone calls, emails, etc. with microsoft we got a solution.

For IMAP you need to use the shared mailboxes email address.
For SMTP use your users email address
Token is the same for both!

[p3root]

Make sure to have SMTP Authentication enabled. Can also be that the shared mailbox needs to have a license. but I could not verify this right now. Updates pending!

[jstedfast]

For IMAP you need to use the shared mailboxes email address.

So would you use "[email protected]", for example? Or would it be "[email protected]\\sharedMailbox"?

[p3root]

You need to use [email protected]