Attachment of Email (...) to record failed due to Message not found
Steps To Resolve Error
Step 1: Make sure that the user is not logged into multiple Gmail Accounts on a single browser.
Step 2: If they are, ask them to logout of everything.
Step 3: Then, first log-in using their CloudExtend Gmail email account.
Step 4: Once that is done and attaching messages to records has tested successfully, then you can now log-in to the other Gmail accounts.
Step 5: If you are using Chrome, you can also create a new profile that your CloudExtend Gmail account can solely use. You can do this by going to settings and clicking "add new user".
This limitation is there because of limited support for multiple logins by the Google Apps Script.