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Sent baseball cards to Sandy Koufax, Graig Nettles, and Dennis Eckersley attention their respective team stadiums and received all three back refused/return To sender. I know that Koufax does not sign (although I have heard of some minimal successes so I that I'd take a chance), I'd previously tried Nettles during his playing days and received a pre-print (I thought that maybe he'd sign since he's been retired for some time), and this was my first time trying Eckersley.
I assume that since letter was stamped 'refused' and most (if not all) stadiums forward their mail to their former players, these three particular players do not sign.

Very few stadiums or teams forward to people no longer with the organization. If that former player happens to be on the coaching staff currently, it will get sent to them, but not if they're retired and have no affiliation with the organization.

sorry for the failures

I've had success with Nettles a few years ago. Maybe he's changed his signing habits since then. The other two you will probably never get TTM.