Mail.app: Cannot remove attachments from sent emails #

If Message > Remove Attachments is grayed out for sent messages:

Select the affected mailbox (e.g., Sent)
Mail > Preferences... > Rules > Add Rule
Create a rule in which From is equal to your email address and Any attachment name contains "." (without quotes).
Set the action to "Set Color"
OK > "Do you want to apply your rules to messages in selected mailboxes" > Apply

Attachments can now be removed normally from affected messages. Tested in Mail 9.3 under OS X 10.11.6. Lion/Mountain Lion's Mail: Workaround for removing attachments from sent messages should have been in the top 10 Google results but wasn't, hence this recapitulation.

/mac | Nov 24, 2016