Winmail.dat showing up as attachmentsIssue Email messages sent from Exchange use the Transport Neutral Encapsulation Format (TNEF) format. Messaging systems may not be able to interpret the messages that use this rich text format. If the recipient's messaging system can't process this format, a file attachment that's called Winmail.dat is added to the message.ResolutionTo change the message format for an external contact that was added to Exchange Online, follow these steps: Connect to Exchange Online by using remote PowerShell. For more info about how to do this, go to the following Microsoft website: Connect to Exchange Online using Remote PowerShell Run the following Windows PowerShell commands to configure the message format as Text Only: Set-MailContact <ExternalEmailAddress or GUID> -UseMapiRichTextFormat Never Set-MailContact -Identity <ExternalEmailAddress or GUID> -UsePreferMessageFormat $True Run the following Windows PowerShell command to confirm that the message format was applied: Get-MailContact | Select <ExternalEmailAddress or GUID> | Select UseMapiRichTextFormat Related LinksMicrosoft website instructions