I am trying to alter the Gmail subject by prefixing it with certain word if it matches the filter condition. I am aware that it is not possible to modify the subject line using Google App Script. But is it possible to modify it using GMAIL API and App Script. Also, I am member of G-Suite so is there any default configuration or inbuilt method to achieve the same.
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It is technically not possible to modify the subject or content of an email message via the Gmail API. You can only change the labels via the API.
One possible workaround is to retrieve the original message, change only the subject line and append the message in the Gmail thread with Insert.