tcp port 645 - smtps?

[Robert Moskowitz]

I see a rule on my firewall defining a custom port of 645 as smtps, yet 
I cannot find anything at iana on this and googling does not explain 
it.  'netstat -na|grep 645' does not show anything listening on this 
port.  No telling how many servers ago this rule got added.

Is this some old bogus holdover, or something I should be using?
Still trying to figure out why yahoo won't email me...