▁joined ▁the ▁Universal ▁Post al ▁Union ▁( UP U ) ▁on ▁ 1 9 ▁May ▁ 1 9 2 2 . ▁In ▁addition ▁to ▁the ▁ordinary ▁mail ▁( based ▁on ▁road ▁transport ), ▁Est onia ▁also ▁had ▁a ▁ship ▁and ▁air ▁mail ▁services . ▁Naval ▁transport ▁was ▁used ▁for ▁sending ▁mail ▁to ▁Hels ink i ▁and ▁Stockholm . ▁In ▁ 1 9 2 3 , ▁ ▁Aer onaut ▁Airlines ▁began ▁to ▁carry ▁mail ▁six ▁times ▁a ▁week ▁to ▁Hels ink i ▁and ▁R iga . ▁Before ▁the ▁start ▁of ▁the ▁Second ▁World ▁War ▁( in ▁ 1 9 3 9 ) ▁and ▁the ▁Soviet ▁an nex ation ▁( in ▁ 1 9 4 0 ) ▁a ▁total ▁of ▁ 1 6 3 ▁st amps ▁and ▁ 4 ▁stamp ▁blocks ▁were ▁put ▁into ▁circul ation . ▁ ▁The ▁development ▁of ▁the ▁Post al ▁Service ▁ ▁By ▁ 1 9 3 5 , ▁the ▁Post al ▁Administration ▁earned ▁already ▁a ▁profit ▁of ▁ 1 . 5 ▁million ▁k ron es ▁with ▁more ▁than ▁ 3 , 6 0 0 ▁out lets ▁in ▁Est onia ' s ▁territory ▁for ▁the ▁provision ▁of ▁postal , ▁ 7 8 2 ▁ones , ▁of ▁te legraph , ▁and ▁ 1 . 8 4 1 ▁ones , ▁of ▁tele phone ▁services . ▁The ▁service ▁network ▁included ▁ 1 2 0 ▁combined ▁post - te legraph - tele phone ▁offices , ▁the ▁rest ▁of ▁the ▁system ▁consisted ▁of ▁auxili ary ▁units ▁– ▁ 6 0 0 ▁postal ▁ag encies ▁and ▁nearly