On Monday, 21 November 2016 10:34:58 UTC, The Natural Philosopher wrote:
An elderly friend is going abroad over Christmas for 5 weeks to visit
family, and wants to know what to do about his letterbox, which wont
hold 5 weeks of mail and would rather give the game away if left stuffed
with weeks old letters.

Poste Restante seems to only hold mail for two weeks.

What are the alternatives?

He also needs to turn off water and drain down tanks in loft.
Including central heating.
Also gas & electricity.

Empty freezers & fridges.
Get rid of perishable food.
Clean and leave doors open.

Hide/remove all valuable items.

Leaving houses empty this time of year is bad news.

Get someone to go in every few days & check the place out.