I wonder what the "exceptional reasons" are?

I know I've said it many times before, but whenever a question is asked by an MP in Parliament the instant thought I usually have, especially if it sounds slightly unusual, is why did they ask that?

Such a question came from Dianne Abbott yesterday when she asked the Home Office if correspondance between MPs and prison inmates "is routinely opened by HM Prison staff before it reaches recipients". In response, Gerry Sutcliffe said,

"Correspondence between Members of Parliament (acting in their constituency capacity) and prisoners is not routinely opened by prison staff. Under confidential access arrangements operated by the Prison Service such letters are not opened, read or stopped unless there are exceptional reasons for doing so.

Got that? Basically "we don't 'routinely' do it but we do do it". Now, I wonder what the "exceptional" reasons might be that means the Home Office authorises the reading of MPs correspondance?

I'd presume that "exceptional reasons" might be where they think information relating to a crime or perhaps national security might be contained within the correspondance? In which case has their been correspondance read on such grounds, and of whom?

All very curious, I'm sure there's a perfectly reasonable explaination.