Hi @Marie Swo ,
Did you flag the email after you send it? If so, it just flags the email for you.
Beside, if you want to flag the email for recipient, you must set the flag before you send the message.
Refer to the steps:
1.On the Message tab, in the Tags group, select Follow Up, and then select Custom.
2.Select Flag for Recipients.
Additional documentation can be found here: Flag email messages for follow up
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