(ordered by popularity)
electronic
direct
registered
next
class
regular
outgoing
certified
incoming
daily
courier
ordinary
chain
postal
unsolicited
snail
weekly
unopened
express
voice
overnight
unwanted
fast
bulk
monthly
forward
interoffice
diplomatic
overseas
air
outbound
encrypted
inbound
anonymous
secure
undelivered
unanswered
congressional
down
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