At the risk of diverting into an investing thread, did you do a mega back door Roth in 2020? I received the same email with an amount that is roughly 2% of my 2020 MBD Roth IRA. My only guess is they are collecting dues on the 401k excess that was self generated.
IIRC we received communication from DALPA that we wouldn't pay dues on self induced 401k excess pay. The dues would only be collected once exceeding the 415C dollar limit of company contributions.