Your comment "if I do not enter into a retail contract with them" leads me to ask have you moved into this property recently ?
A rental ?
If so, Sumo were likely the last tenants power retailer, and if the old tenant didn't request a final read, and you didn't request a new service from AGL (and a meter read to get the new service starting from X reading), then the old tenants supplier maintains supply to the property.
This will also lead to them getting bills to the address or another like a PO Box, and if they don't pay, it could possibly have ramifications for you, you might be responsible and affect credit scores in some way, especially if you are partly responsible for power use at the property after X date.
If this is just the same old place you lived for a while, you have been with AGL, and been getting AGL bills, then these might be scam letters of some sort, or Sumo have got the address mixed up.
It could all come down to circumstance regarding how long at property, if you are still a current AGL customer, but I would call Sumo and ask if they have the details right in either case, and work through a solution to end any possible old tenants info over the property.
