An Error occurred, but the error response could not be deserialized
you might be perplexed, since the error doesn’t provide any actionable information. This is an error that is generated by the Gmail API when GMass connects to your Gmail account to send your mail merge campaign. While there’s not much information available from Gmail as to the cause of this error, we’ve found that the usual cause is the user of large attachments or inline images.
Gmail states that emails can be up to 25MB in size, and GMass places a further size limit of 12MB per individual email message, but still, Gmail may generate this error even if you are under both of these limits. The solution is to simply reduce the size of your email, either by shrinking your inline images, host your images and refer to them relatively in your HTML code, or by reducing the size of or eliminating attachments.
This Stack Overflow article corroborates our theory about this error.