This is a contact form ASP.NET Core 3.1 application. E-mail messages are not sent right away, but they are pushed to a queue managed by an ASP.NET Core Hosted Service. This means the client won't have to wait for the actual delivery. Each message is then dequeued and delivered in background. In case of an error during delivery, a message is re-queued for later processing.

Check out the actual Hosted Service implementation in the code file HostedServices/EmailSenderHostedService.cs.

The Hosted Service has to be registered int the ConfigureServices method of the Startup class. Take a look at the Startup.cs file.

This project uses the MailKit package to deliver emails. The Hosted Service implements the IEmailSender interface from the Microsoft.AspNetCore.Identity.UI but you can make it implement some other similar interface if you don't want to depend upon ASP.NET Core Identity.

This code is NOT production-ready. It just illustrates how to implement and register a Hosted Service for sending e-mails in background. Some messages might be lost when the application is restarted since it does not implement a persistent storage (e.g. a database).

See this fork by AngeloDotNet for a more complete implementation using persistent storage (Sqlite). https://github.com/AngeloDotNet/BackgroundEmailSender

Moreover, this project it does not deliver email messages in parallel or throttle them to prevent reaching a quota.

Edit the appsettings.json file with your SMTP server data. Then, just run the application by typing dotnet run. The .NET Core SDK 3.1 (or greater) must be installed in your system. Fill in the form and hit the Send button.