Send-MailMessage : Mailbox unavailable. The server response was: 5.7.60 SMTP

Hi all,

This forum really is my last option, cause I checked in the internet for a solution and was not able to find.

I have a hybrid exchange scenario - Exchange2019 + O365 - all the mailboxes are remote mailboxes (hosted in Office365) and we use Exchange2019 only to manage the features and SMTP Relay.

The point is that I am trying to send email from an specific mailbox using smtp relay and I am facing the issue below:

Send-MailMessage : Mailbox unavailable. The server response was: 5.7.60 SMTP; Client does not have permissions to send as this sender

The .ps1 script that I'm using is below:

[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12

       $sendMailParams = @{
            From = "donotreply @ mydomain.com"
            To = "myname @ mydomain.com"
            Subject = "Test mail from $env:Computername"
            Body = "<p> *email sent for tests purposes* </p>"
            SMTPServer = "smtp.mydomain.com"
            Port = 587
            Encoding = "UTF8"
        }

        Send-MailMessage @sendMailParams -BodyAsHtml -Credential $credential -UseSsl

After running the script, obviously I need to insert the credential then get the mentioned error.

Other details regarding my tests:

• User inserted into '-Credential $credential' part is something like "test@ mydomain.com"
• The receive conector is allowing the IP source
• The receive connector is configured to listen port 587
• The receive connector is configured for SSL, Authentication and Exchange Users as well
• The relay must be done using SSL and Authentication (I can not use Anonymous)

The user test@ mydomain.com is already added as SEND AS into donotreply@ mydomain.commailto:email address removed for privacy reasons

Any ideia what I should do to allow the test@ mydomain.com to send emails as donotreply@ mydomain.com using smtp relay?

Thanks in advance.