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: https://github.com/jstedfast/MailKit/issues/1412#issuecomment-1196872919

[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 "sharedMailbox@outlook.com", for example? Or would it be "username@outlook.com\\sharedMailbox"?

[p3root]

You need to use sharedMailbox@outlook.com