The most common cause for an "Error" message when attempting to send an email is due to image size. Messages should not be over 262144bytes (.26MB) in size.

The error can apply to the image(s) you are attempting to use in the body of the email or in your signature.

The easiest way to ensure your images will not affect your emails being sent is to use a free image compression website.

We recommend TinyPNG.

As for your signature, it's generally recommended that it not be greater than 1000kb in size for deliverability purposes regardless of whether it is being used in Shaker or elsewhere.

Here's a great blog post with more information: Creating a killer real estate signature