New Default System Address Rule for Email

[les]

Hi,

I would like to have a default system address rule for email of...

<first><first 3 chars of surname>@mydomain.com

i.e. for John Smith it would look like...

johnsmi@mydomain.com

The closest ive seen is johns@mydomain.com.

I cant seem to find any suitable syntax for a command such as sxconfig to grab 3 characters of a surname. Its seems like the first letter or the entire last name.

Is it possible?

TIA