FIXED – Why is Outlook sending Winmail.dat to Mac Mail clients – the Autocomplete cache was the reason for me

The Fix (for me)

If you have tried all other suggestions (see below), then try deleting your Outlook Autocomplete cache:

  1. Either 1 at a time by starting to type name with problems in the “To” field and then click the “X” against the address with the problem
    Outlook auto cache1
  2. Or to delete the whole cache (the “to be completely sure” option!!) then: Outlook File tab, Options, Mail, Send Messages, Empty Auto-complete List
    Outlook auto cache2

Why does this work?

There are lots of threads about emails sent from Outlook with an attachment (e.g. photo) being received by Mac Mail with the attachment replaced by a file called winmail.dat.  And there are lots of places offering essentially the same answer as this very comprehensive one from Slipstick ( In summary you need to ensure that the sending format is NOT set to “rich text” in multiple locations:

  1. Specific contact card details for recipient
  2. General sending options in your outlook client
  3. Options on the Exchange server (inc O365)
  4. Etc etc

The suggestions here may fix the issue for you, however even after trying everything the problem was still there for me.

After the normal search, search and search again using different key words I came across a discussion about the Outlook Autocomplete cache and how this stores a snapshot of data about the contact separately.  So I searched for a tool ( that would allow me to read the internals of the .NZ2 file and see what data is stored.  And as you can see from the results below there is a field “Sending Format”.  After clearing the Autocomplete cache and trying to send again, viola the problem was resolved:



Address Type:

Display Name:

Email/Exchange String:

Drop-Down Name:

Search String:


SMTP Address:

Display Type:

Record Weight:

Sending Format:


Linked To Profile:




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