Visual Studio .NET - automatic email sending in asp.net using c#

Windows Service[^] is the best choice.
Refer more: http://msdn.microsoft.com/en-us/library/d56de412(VS.80).aspx[^]

Sending Automated Emails asynchronously using a C# Windows Service in conjunction with Database Email records – Part I[^]
Sending Automated Emails asynchronously using a C# Windows Service in conjunction with Database Email records PART – II[^]

Alternatively, you may use Task Scheduler (Windows)[^]
A comparison about Task Scheduler vs Windows Service[^]

Or use SQL Server jobs[^]
SQL Database Mail - Send Emails from SQL Server[^]
SQL Database Mail - Send T-SQL Results by Email[^]

BTW found a nice article in CP(Nice article)
Simulate a Windows Service using ASP.NET to run scheduled jobs[^]

Check out CodeProject Answers[^] for similar question asked before.

You can use the

System.Net.Mail

Namespace if you have a SMTP-Server at hand.