Introduced a new project, `DigitalData.MessagingService.Publisher.Abstraction`, to encapsulate the `OutgoingEmailEvent` class and `IOutgoingEmailPublisher` interface. The project targets multiple frameworks (`net462`, `net480`, `net8.0`) and enables nullable reference types and the latest C# language version.
Moved `OutgoingEmailEvent` and `IOutgoingEmailPublisher` to the new project, updating their namespaces and replacing `required` properties with mutable ones in `OutgoingEmailEvent`. Updated all dependent files to reference the new namespace.
Updated the solution file to include the new project and adjusted build configurations. Cleaned up unused `using` directives and removed redundant `LangVersion` property in the new project file.
The `MimeKit` package reference (version `4.17.0`) has been removed from the `DigitalData.MessagingService.Application.csproj` file. This change indicates that the project no longer relies on the `MimeKit` library for its functionality.
