In my own testing, the only way I could get the SME, in "server only", to pass other servers through, was to install a contrib by Darrell May called:
dmc-mitel-proxypass-0.0.1-3.noarch.rpm
It seems that when the SME is set as "server only", it will not do the routing(?).
Although when set to server "gateway" it has always worked properly.
Good luck
Joe