Access back-end messages (not the one from the event)

I once wrote a GMail widget that I had to convert to a Chrome add-on when Google removed support for the GMail widget, and I'm now trying to convert it to a Gmail Add-on.

My issue is that my add-on loops thru a series of messages under a label and tries to reply to them, but I get a error trying to access them:

ERROR: Exception: Access denied: : Missing access token for per message scope authorization.

So my question is, how can I access those messages? The only access token I have is the one that triggered the add-on and that won't do.

Or asking from a different point of view, are there more events that can trigger the add-on besides opening a message?

Thanks in advance.

BTW, the Chrome extension that I'm trying to convert to a Gmail add-on can be seen here:

https://sites.google.com/site/replytomany/home

https://chrome.google.com/webstore/detail/reply-to-many/gpmpcjkhfjflmjpjjmdegpkgginijbin?hl=en

[EDIT] This is what my appsscript.json looks like:

{
  "timeZone": "Europe/Dublin",
  "dependencies": {
    "enabledAdvancedServices": [{
      "userSymbol": "Gmail",
      "serviceId": "gmail",
      "version": "v1"
    }]
  },
  "oauthScopes": [
            "https://www.googleapis.com/auth/gmail.addons.execute", 
            "https://www.googleapis.com/auth/gmail.readonly",
            "https://www.googleapis.com/auth/gmail.send"
         ],
  "gmail": {
    "name": "Reply To Many",
    "logoUrl": "https://www.gstatic.com/images/icons/material/system/1x/receipt_black_24dp.png",
    "contextualTriggers": [{
      "unconditional": {
      },
      "onTriggerFunction": "getContextualAddOn"
    }],
    "primaryColor": "#41f470",
    "secondaryColor": "#94f441",
    "version": "TRUSTED_TESTER_V2"
  }
}

Solution

Did you try implementing the access token? I meant for you to try that first as I believe that's your issue if you don't have it.

// Activate temporary Gmail access token. Where 'e' is the function argument
var accessToken = e.messageMetadata.accessToken; 
GmailApp.setCurrentMessageAccessToken(accessToken);

if you have "mail.google.com/", then you don't have a scope problem, as that gives you full access. The only other security issue is the access token.