Email approval Button missing in Hybrid Scenario (Complete solution with troubleshooting)

In pure cloud or pure on-premises environment email approval works fine without any error. But in Hybrid scenario it’s very difficult to make sure email approval works when users are in cross premise environment. If you look for Microsoft Doc or any other third-party document, you won’t find any proper document. Here I am sharing one full step including all kinds of troubleshooting.

The key and only factor of working with email approval is TNEF (Transport Neutral Encapsulation Format). TNEF, also known as the Transport Neutral Encapsulation Format, Outlook Rich Text Format, or Exchange Rich Text Format, is a Microsoft-specific format for encapsulating MAPI message properties. All versions of Outlook fully support TNEF. Outlook on the web (formerly known as Outlook Web App) translates TNEF into MAPI and displays the formatted messages. Other email clients that don't support TNEF typically display TNEF formatted messages as plain text messages with Winmail.dat or Win.dat attachments.

Step by step demonstration to resolve email approval issue:

The email approval button was missing because you have disabled TNEF or TNEF is not enabled. To enable TNEF log into your exchange server and run two PowerShell command in your exchange management shell.

Get-RemoteDomain | Select Name, DomainName, TNEF*, Trust*, AllowedOOF*, IsIn*      [To check ]

Get-RemoteDomain | Set-RemoteDomain -TNEFEnabled $true    [To change]

After you change the TNEF you will see Approval button reappeared in your outlook. But it will create you another problem. Your email will start getting Winmail.dat as attachment.

To resolve the issue, you have to change the setting from your ECP. You are receiving Winmail.dat  because you made the change only from exchange on premise side. You did not changed some from office 365 side. You have to make sure Rich text format is disabled or you can use HTML or plain text. Because not everyone can read or convert rich text format.

• Login to office 365 admin portal and navigate to exchange admin center (convert to classic exchange)
• Select mail flow
• Inside mail flow select remote domain
• Select Default one and make necessary changes (Screenshot-1)
• If you have only one create a new one for your custom domain
• Click plus and keep everything same except same option (Screenshot-2)
• In domain name place use *.contoso.com

After everything setting properly sometimes you might see some approval might fail with a NDR message

To resolve the issue, add all the on-premise system mailbox in office 365 as mail contact.

First find system mailboxes from on premise using below command. Login to on premise exchange management shell and run below command

Get-Mailbox -Arbitration | Fl Name, DisplayName

Add them into office 365 as mail contact following below format.

Format: SPTP:Email address removed

Replace System mailbox value with the value you found from on premise

Replace custom domain with your domain name.

Example: SMTP:SystemMailbox{2CE344-31E-D-9D7-A7C7D7A0DAA}@contoso.com 

Give first name, last name, and display name whatever you want. Just make sure you add email address according to format.  

• Go to Office 365 exchange admin center
• Go to recipient
• Go to contact
• Click add contact