A timeout generally means that the SmtpClient was either unable to connect to the SMTP server or the connection was severed or lost at some point during the send operation.
I would suggest going back to ‘Tools > Settings > Email notifications’ and testing your email configuration with the ‘Test email settings’ button to make sure emails are able to send successfully. Your issue could be caused the SMTP server being unavailable or unresponsive at the time of error, or it could be caused by a firewall on that is blocking outgoing connections to the server or blocking outgoing connections on whichever port you are using etc. I believe we have even seen a timeout occur as a result of the mail server limiting the size of attachments to a smaller value than the attachment size that is attempting to be sent.