You can decode base64 encoded email messages in Gmail, or files in your Google Drive, using the base64Decode() method of Google Apps Script.
function base64DecodeEmail() { // If the base6f.txt file in Drive needs to be decoded var mime = DriveApp.getFilesByName("base64.txt").next().getBlob().getDataAsString(); // If a Gmail message with subject base64 needs to be decoded var message = GmailApp.search("subject:base64")[0].getMessages()[0]; var mime = message.getRawContent(); // Concatenate the chunks in the source mime = mime.replace(/\n|\r/g, ""); // Extract the base64 string using regex var matches = mime.match(/base64[a-zA-Z0-9\+\/=]+/g); for (var line in matches) { var base64 = matches[line].replace(/^base64/, ""); var decoded = Utilities.base64Decode(base64); var html = Utilities.newBlob(decoded).getDataAsString(); Logger.log(html); } }