Are there any attachments to the Send Email action when it fails at the end of your build?
When you say that running the action alone it works, are you running the same action (with all the same settings, message and attachments) or did you create another send email action to test this? The fact that the action works when run on its own suggests, there is a difference in the configuration somewhere.
This error message occurs when the message you are attempting to send, exceeds the the message size limit configured on your email server. This size limit includes headers, text and attachments.