Using the diplomatic pouch

Hanbury-Williams found himself in the topsy-turvy position of being expected by London to further the interests of Britain’s new ally, Frederick of Prussia, whom he had originally been sent to Russia to undermine. In this way, the grand reversal of alliances among the European powers was mirrored in miniature by the reversal Hanbury-Williams was forced to make in his own objectives and efforts in St. Petersburg. The Englishman did his best. He became a diplomatic acrobat. Frederick had no envoy in St. Petersburg; Hanbury-Williams secretly offered to take on the role himself. By using the diplomatic pouch destined for his colleague the British ambassador in Berlin, he would endeavor to keep the Prussian king informed of what was happening in the Russian capital.

Could anyone explain the meaning of 'pouch' or 'diplomatic pouch'? I don't find a suitable entry for it.

Thanks!