We should consider to separate Contracts (Repositories & Specifications) by Entity.
Instead of having a single namespace like:
Microsoft.Samples.NLayerApp.Domain.MainModule.Contracts
And all the contracts classes within it. We could have the following namespaces:
 
Microsoft.Samples.NLayerApp.Domain.MainModule.Contracts.Customer
Microsoft.Samples.NLayerApp.Domain.MainModule.Contracts.BankAccount
 
Then, just the contracts for each entity within each..
 
Cesar.