Open Mailbox With Raised Flag on Reddit

What it signals here

upvote context

Used in comments to signal someone has been sent a DM or that important info is in their inbox.

community use

In communities like r/penpals or r/snailmail, used literally and affectionately.

subreddit meaning

On advice or help subreddits, used to indicate a mod or commenter has messaged the OP directly.

karma context

Occasionally used humorously in karma farm posts with a 'your reward is in the mail' joke.

See it elsewhere