Groovy REST sendmail fails with "IOException while sending message"

Summary:

Groovy rule to sendmail via REST. The request works fine in that it is passed through the connection. But the email never arrives. Checking the status returns status:1 and details: IOException while sending message

Content (required):

Can send the email with REST using postman but cannot get the groovy rule to suceed. Are there any error code references and or debugging resources for this? The migration status code just says: +ve "Operation failed, with the status signifying an error". Not useful.

Any help is appreciated.

Version (include the version you are using, if applicable):

PBCS

Code Snippet (add any code snippets that support your topic, if applicable):