JKB, what about pouring some concrete, installing a short section of pipe with a collar on it, and a corresponding collar on the pipe that is attached to the mailbox, using two 1/4" Grade 2 bolts? They'd shear off pretty quick if hit.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).