If it says 18mb free, that's the amount of white space after the last online maintenance.
That means an offline defrag is a very bad idea as it will take absolutely ages and free up nothing.
You should reduce retention, have a massive clearout and then if you are ok for disk space and backup window time, don't do an offline defrag as exchange uses the logical size not the physical size.
The logical size is physical size of DB+STM - White space in 1221.
Shaun
