Hello,

I just set up oAuth2 for one of our email addresses. I am successfully authenticating to it because email fetching is working, however I get errors when attempting to send mail out.

Unable to email via SMTP: helpdesk@xxxxx.xxxx (tls://smtp.office365.com:587/SMTP) 5.7.3 Authentication unsuccessful [MW4PR03CA0088.namprd03.prod.outlook.com]

Unable to email via Sendmail Unable to send mail: Unknown error

Unable to email via SMTP: helpdesk@xxxx.xxxxxx (tls://smtp.office365.com:587/SMTP) Cannot issue HELO to existing session

Unable to email via Sendmail Unable to send mail: Unknown error

I've verified the correct permissions are set in azure according to the oAuth2 guide and the servers match this https://support.microsoft.com/en-us/office/pop-imap-and-smtp-settings-8361e398-8af4-4e97-b147-6c6c4ac95353

SMTP was working on this using basic authentication earlier today.

I want to add that these forums have been a massive help through this whole process and thank you to everyone who knows more than me for answering so diligently. Enabling URL rewriting on the server should probably be in the oAuth documentation as a note, that one took me awhile to troubleshoot but the answers were here on the forums!

    TBCS2

    The authentication is failing for SMTP so you need to investigate that part. Why are you not using OAuth2 for SMTP? You can set Authentication to Same As Remote Mailbox and it will use the same token, etc. configured for fetching. You can also try reviewing older discussions here and try those suggestions. If all else fails contact MS to see if they can investigate the authentication failure.

    As for Sendmail that’s used when you don’t configure SMTP or when SMTP fails as a last resort. That’s giving an unknown error so I’m unable to diagnose why that’s failing.

    Cheers.

        KevinTheJedi Sorry if it wasn't clear I do have "same as remote mailbox" checked. That's why I thought the error had to be something with my configuration since clearly the authentication aspect is working since it can fetch emails.

            TBCS2

            For that error I've seen people delete the email in MS and re-add it and it works afterwards. This is most likely due to when MS was migrating accounts to modern auth they messed something up for some accounts and it's stuck on something. Some people have also disabled SMTP and re-enabled it. Like I said, if all else fails, contact MS for further assistance.

            Cheers.

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