My comments

  • removed IConfiguration from library - should be handled in application layer, to be resolved by DI container, based on that, proper class should gets injected to service, added TransferServiceConfiguration transferServiceConfiguration - as an example what could be used by DI (or purely by config.Bind)

  • added Abstract folder to keep interface - readability (some abtsractions could be even propagated

  • extracted IContainerDataStore in order to respect that one by dedicated data stores implementation

  • renamed Types to Models since it collides a bit with conventions, enums kept in dedicated folder

  • moved all services to one subfolder - readability (to avoid confusion when logic resides)

  • mark some models props with null! to mark which reference nullable types are nullable or not

  • MailTransferService

  • service should operate based on abstrations, DI should be smart enough to inject implementation based on configuration, SOLID - dep. inject prinicple

  • removed request validation part from service ( in that place it is assumed models were validated before), we follow SOLID - single resposiblity, validator could be invoked diffrent way (like middleware in api), thus I removed it from service, added dedicated class which is based on fluentvalidation added another one which could handle it in custom way

  • added try catch withh Ilogger to add more light into app

  • Tests

  • added test for service with mocked dependency interface by moq

Future improvements -

  • making signature to support working with tasks (helpful later in impl to make whole process async)
  • more tests for dedicated services, also validator (now added happy path)
  • respect CQS - split data store logic to have either query or command
  • more validations needed since it also is based on object retrieved from container (dynamic validation)