Herbie and mole man are the biggest guesses online right now. Others think willie.
Wasn't he a comic strip by stan lee before fantastic four was made?A quick read up on the Wiki, He doesn't do the mail as much but I think still did the occasional odd job for the group as late as the Slott run, such as helping run their letters page and some stuff with the future Foundation.
His niece also did the mail for a while after they came back during Heroes Reborn, but that character seems to be mostly in limbo.
Edit--yup.
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