Also know, do emails have a unique ID?
Message-ID is a unique identifier for a digital message, most commonly a globally unique identifier used in email and Usenet newsgroups. No two different messages must ever have the same Message-ID. If two messages have the same Message-ID, they are assumed to be the same and one version is discarded.
Beside above, what is message ID Flow? The Message Id field of the "Forward an email" action is required to provide a Message Id (the unique identifier of the Outlook message) of an Outlook email rather than the Dynamic contents that you selected from the trigger (the trigger from OneDrive connector or OneDrive for Business connector).
Consequently, where is the message ID in Outlook?
Message-ID of an email can be found in the message header.
How do you personalize your email in Outlook?
> View all Outlook settings. Select Mail > Compose and reply. In the text box, type your email signature. You can change the font, size, color, and the appearance of your signature from the options available in the ribbon on top of the text box.