Unfortunately, OFT-files are meant for MS Windows only. And currently they cannot be used on Mac OS.

Please use MS Windows to open OFT files or pick any other exporting method

Also, as an option, you may send a test email to Outlook which you use on Mac OS and send over to recipients. 

Please be advised that your email may look somewhat different from you planned due to email client’s rendering issues. 

Today, this is a well-known problem. Please refer to Microsoft’s official website to find answers from this email client. 

https://answers.microsoft.com/en-us/msoffice/forum/all/how-do-i-open-an-oft-file-on-mac/e6999adb-da9b-4487-bdb2-eaa2d712a10c 

And also to see users’ comments, please visit this forum on Outlook official site:

https://outlook.uservoice.com/forums/293343-outlook-for-mac/suggestions/10181397-open-oft-file-on-outlook-for-mac

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