When external recipients complain about receiving attachments in winmail.dat format ( or sometimes ATTxxxxx) this is most likely caused by the formatting. To be more concrete it is caused when Exchange uses RTF ( RichTextFormat ) for sending mails to external recipients.
To solve the winmail.dat issue on serverside the external recipients should be added as Mail-Contacts to the Exchange organization. You can force Exchange to never use RichTextFormat when sending mails to these Mail-Contacts.
To force Exchange to never use RTF format when sending to external contacts use :
Exchange 2010 Multi-Tenant : ( for a certain Mail-Contact )
Get-MailContact -Organization “TenantName” -Identity “Mail-ContactName” | set-mailcontact -usemapirichtextformat never
Exchange 2010 Multi-Tenant : ( for all Mail-Contacts in an organization )
Get-MailContact -Organization “TenantName” | set-mailcontact -usemapirichtextformat never
Exchange 2013 / 2016 Multi-Tenant : ( for a certain Mail-Contact )
Get-MailContact -OrganizationalUnit “TenantName” -Identity “Mail-ContactName” | set-mailcontact -usemapirichtextformat never
Exchange 2013 / 2016 Multi-Tenant : ( for all Mail-Contacts in an organization )
Get-MailContact -OrganizationalUnit “TenantName” | set-mailcontact -usemapirichtextformat never
For Exchange OnPremise you should most likely use these CMDlets without the -Organization or -OrganizationalUnit switch.
You can also consider to use the Remote Domain solution.
Since we only have Multi-Tenant environments we want to control this on organization level. For this reason we did not mess with the Remote Domain. Hence we have not tested it. But a comment on this from someone , who has implemented the Remote Domain solution in a Multi-Tenant environment , would be well appreciated.
This will solve the problem on serverside. Attachments will not longer appear as winmail.dat files.
See a client side solution here :
Winmail.dat Attachment Outlook