I have upgraded to Sierra and lost the ability to "Remove Attachments" before I file the message. The command appears but is shaded grey and unusable. Anyone else had this problem?
contains results of a google search for "Mac OS Sierra Problems". The first one mentions a few "things" about Apple's Mail program, but not the specific issue you are having.
Also, there is an update for Sierra, OS 10.12.1, that is currently in beta testing, and it should be released by the end of the month.
Sorry I can't help with it, as I use Outlook 2016 for my EMail needs.
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