Hi @Anonymous ,
Welcome to Microsoft Q&A forum and posting your question here.
According to my test, the result is as follow:
- For the sent message, it is not feasible to remove flag for recipient.
- For the new message you are composing, it is feasible to do so.
On the Message tab, in the Tags group, select Follow Up > cusomize > Uncheck the box "Flag for Recipients"
Besides, here is an article about flag enails for your reference.
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