I am trying to add an additional UPS account to ShipRush. I have 3 accounts that are working fine, but the 4th account that I am trying to activate is stuck at Account activation is pending.

I processed a test shipment on the main account, voided the shipment and tried running end of day. I have the following error in ShipRushServer.log:
10/19/2016 6:42:00 AM ERROR Send Mail transaction failed: Error 50510 (details: Unknown user [5001])