Recently I was asked how to respond immediately to 3 000 letters. It was obviously stupid to open each letter separately.
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Batch Reply for GMail will help massively respond to emails in Gmail.
After installing the extension and selecting the necessary letters, a new Batch Reply button will appear in the Gmail panel.
When you click on it, a window for creating a new letter will open, where all the recipients will be in a blind copy.
Unfortunately, you cannot reply within the selected letters. All recipients will receive a new letter as part of the new correspondence. The fact that it was a mass mailing they will not be able to find out. Bcc addresses are not visible to anyone except the sender.