Client does not have permissions to send as this sender

Question

Hi All,

I am using an Exchange 2016 Hybrid environment with an internal relay. When I test using the syntax below, I receive the following error. For authentication, I am using a service account that is created as a remote mailbox on-premises and synced to Azure AD, which also has an Exchange Online Plan 1 license.

smb@contoso.com is a shared mailbox created as a remote shared mailbox.

onlinedl1@contoso.com is an Exchange Online distribution list.

I have assigned both the service account and the shared mailbox Send As permissions on the distribution list.

Send-MailMessage -SmtpServer mysmtp.contoso.com -Credential $credential -From 'smb@contoso.com' -To 'onlinedl1@contoso.com' -Subject 'Test' -Port 587 -UseSsl  

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

Answer 1

Hello, @Glenn Maxwell,

Welcome to the Microsoft Q&A platform!

Based on your description, I understand that you got the error message “Mailbox unavailable. The server response was: 5.7.60 SMTP; Client does not have permissions to send as this sender.”  when using PowerShell for syntax testing.

According to the error message, it’s indicate that the application is trying to send an email from an address that doesn’t match the logon credentials. For example, if your entered login credentials for User0@contoso.com in your application settings but the application tries to send emails from User1@contoso.com. If your application behaves this way, please use Office 365 SMTP relay because SMTP client submission does not support this scenario. 

It is advisable for you to configure Office 365 SMTP relay If your application does need to send email from a different address. For more, please refer to this document: How to set up a multifunction device or application to send emails using Microsoft 365 or Office 365 | Microsoft Learn.

Please click "Accept Answer" and kindly upvote it if the answer is helpful. If you have extra questions about this answer, please click "Comment".

Thank you for your support and understanding.

Best Wishes,

Alex Zhang

Answer 2

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