I think the fact that Espy outperformed Biden in Mississippi was the most surprising result of this election. The sort of white southern Democrat that used to vote Republican for president and Democrat for senate at this point has been fully realigned to voting Republican at every level and OTOH I'd expect the type of voter realigning towards the Democrats to vote Biden for president and Republican for senate. So who were the nearly 40,000 Mississippians who voted Trump for president and Espy for senate?
Since Trump outperformed previous Republicans among African Americans this year, maybe some African Americans voted this way? Just a total guess.