Crossword puzzle clue "open as mail"

Forgets to mail the mail, say

They come after signatures in snail mail, yet before signatures in e-mail: Abbr.

___ mail (e-mail folder)

___ mail (e-mail alternative)

Like express mail vis-à-vis regular mail

Open, as mail

Rare occurrence at the U.S. Open on Father's Day that's a common occurrence at the U.S. Open on Labor Day

US Open and French Open feats