I set up |/bin/grep Subject|/usr/bin/smbclient -M <netbiosname> as an alias but it doesn't work.

That is a shell command line. Exim does not run pipe commands under a shell by default (for added security - and it saves a process). You need something like

"|/bin/sh -c '/bin/grep Subject|/usr/bin/smbclient -M <netbiosname>'"