Based on your regex pattern, unplugging the cable will not be an adequate test. Your pattern matches on administratively down, so you would need to do a "shutdown" on the port to generate this syslog.
Therefore, the first thing to do is change the pattern to "...changed state to down".
You also used a lowercase 's' in the Serial interface name. Regular expressions are case-sensitive. So you must match the syslog case exactly. This will need to be an uppercase 'S'.
Finally, you should consider using an IP address for your SMTP server. Even if you have DNS configured on the device, there have been issues with stuck lookups. An IP gets around that.
