How/when is defer directory cleaned?

[Francisco Reyes]

I just noticed files in the defer que directory that are over a year old in 
a postfix 2.3.4 installation. Is there a process that is supposed to be 
cleaning that directory?

I looked at a few machines (2.3.4, 2.4.5) and they had simmilar scenarios; 
old files in the defer diretory.