Crossword Puzzle Opening letters

Clue: Opening letters

Memo-opening letters (FYI with 3 letters)
Opening letters? (ABCD with 4 letters)
Opening for letters (MAILSLOT with 8 letters)
Voltage letters (EMF with 3 letters)
City with a landmark spelled out by the circled letters, reading left to right (STLOUISMISSOURI with 15 letters)
Exchanges of letters: Abbr. (CORR with 4 letters)
Letters for sages? (WYES with 4 letters)
Long distance letters (e.g. ATT with 3 letters)
Letters below a 0 (OPER with 4 letters)
Shy person's letters (IOU with 3 letters)