add a parameter to specify an SMTP port number to override default

background:
 - Amazon (may be) throttling traffic on port 25 with their SES leading to connection timeouts for ERT
 - They advertise other ports which are not throttled, but specifying a parameter is required to use them
This isn't really how `@param` is supposed to be used.
I'd instead use `... specified in {@code smtpHost} as the ...`.

But actually, for *private* methods, I don't think javadoc is usually necessary. The code here is pretty clear in what it's doing. (If it wasn't then some javadoc might be helpful.)