Complex sentence: When the letter was sent on August 29,1793, it was Jefferson's intent that it reach Shelby before Michaux did, but Shelby did not receive it until October 1793.
Split into simpler sentences: When the letter was sent on August 29,1793, it was Jefferson's intent that it reach Shelby before Michaux did. The letter was delayed, though, and Shelby did not receive it until October 1793.