The following Message (email/smtp) errors have documented solutions.

Unable to read data from the transport connection: net_io_connectionclosed
This error can occur for multiple reasons, including when platform Messages are sent to SendGrid but the plan limit has been exceeded or a bad API key. The Message is found in the Action Text of the Recipient record.

Message cannot be sent: The operation has timed out.
This error can occur for various reasons when the mail server cannot be reached or is interrupted. A possible culprit is the firewall blocking outgoing connections on the port.

Message does not have 'Subject'. Message body is null.
This error occurs when a Communications_Message ("Recipient" records) with an Action Status: Sent (thus with a null subject and body) was manually changed to Action Status: Ready to Send. 

Message cannot be sent: Error in processing. The server response was: You've exceeded your messaging limits
This error can occur for various reasons. You may have either a daily or hourly limit on your account, or a limit on the number of messages that can go out at one time. Check with your mail processor (usually Sendgrid)