Thanks @Nick_M. That is the process we used to add the Gmail account. We have tried removing it and re-adding it when the issue occurs, but receive the same error.
Again, this is an intermittent issue which occurs and resolves itself without any changes to the Gmail account that we have configured. We can send other emails from within QuickBooks without issue, it seems to only be statements that give us frequent issues.
What additional troubleshooting steps should we take when this occurs?