why dont you go try it out?
mail something to one of the chars on your account, and log on that char, the mail will be there, and log on a different char on the same account, the same mail will still be there.
im not sure whether or not it does the same if the mailbox is not opened where the the mail was sent to.
the only char that can see a new mail is the char that the mail goes to, once you see the mail in the mailbox, it stays there on all 3 characters in the same account until its read or deleted.
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