The SmtpClient class is used to send e-mail to an SMTP server for delivery. The SMTP protocol is defined in RFC 2821, which is available at http://www.ietf.org.

The classes shown in the following table are used to construct e-mail messages that can be sent using SmtpClient.

To construct and send an e-mail message by using SmtpClient, you must specify the following information:

• The SMTP host server that you use to send e-mail. See the Host and Port properties.

• Credentials for authentication, if required by the SMTP server. See the Credentials property.

• The e-mail address of the sender. See the Send and SendAsync methods that take a from parameter. Also see the MailMessage.From property.

• The e-mail address or addresses of the recipients. See the Send and SendAsync methods that take a recipient parameter. Also see the MailMessage.To property.

• The message content. See the Send and SendAsync methods that take a body parameter. Also see the MailMessage.Body property.

To include an attachment with an e-mail message, first create the attachment by using the Attachment class, and then add it to the message by using the MailMessage.Attachments property. Depending on the e-mail reader used by the recipients and the file type of the attachment, some recipients might not be able to read the attachment. For clients that cannot display the attachment in its original form, you can specify alternate views by using the MailMessage.AlternateViews property.

You can use the application or machine configuration files to specify default host, port, and credentials values for all SmtpClient objects. For more information, see <mailSettings> Element (Network Settings).

To send the e-mail message and block while waiting for the e-mail to be transmitted to the SMTP server, use one of the synchronous Send methods. To allow your program's main thread to continue executing while the e-mail is transmitted, use one of the asynchronous SendAsync methods. The SendCompleted event is raised when a SendAsync operation completes. To receive this event, you must add a SendCompletedEventHandler delegate to SendCompleted. The SendCompletedEventHandler delegate must reference a callback method that handles notification of SendCompleted events. To cancel an asynchronous e-mail transmission, use the SendAsyncCancel method.

Note
If there is an e-mail transmission in progress and you call SendAsync or Send again, you will receive an InvalidOperationException.

The connection established by the current instance of the SmtpClient class to the SMTP server may be re-used if an application wishes to send multiple messages to the same SMTP server. This is particularly useful when authentication or encryption are used establish a connection to the SMTP server. The process of authenticating and establishing a TLS session can be expensive operations. A requirement to re-establish a connection for each message when sending a large quantity of email to the same SMTP server could have a significant impact on performance. There are a number of high-volume email applications that send email status updates, newsletter distributions, or email alerts. Also many email client applications support an off-line mode where users can compose many email messages that are sent later when a connection to the SMTP server is established. It is typical for an email client to send all SMTP messages to a specific SMTP server (provided by the Internet service provider) that then forwards this email to other SMTP servers.

The SmtpClient class implementation pools SMTP connections so that it can avoid the overhead of re-establishing a connection for every message to the same server. An application may re-use the same SmtpClient object to send many different emails to the same SMTP server and to many different SMTP servers. As a result, there is no way to determine when an application is finished using the SmtpClient object and it should be cleaned up.

When an SMTP session is finished and the client wishes to terminate the connection, it must send a QUIT message to the server to indicate that it has no more messages to send. This allows the server to free up resources associated with the connection from the client and process the messages which were sent by the client.

The SmtpClient class has no Finalize method, so an application must call Dispose to explicitly free up resources. The Dispose method iterates through all established connections to the SMTP server specified in the Host property and sends a QUIT message followed by gracefully ending the TCP connection. The Dispose method also releases the unmanaged resources used by the Socket and optionally disposes of the managed resources.

Call Dispose when you are finished using the SmtpClient. The Dispose method leaves the SmtpClient in an unusable state. After calling Dispose, you must release all references to the SmtpClient so the garbage collector can reclaim the memory that the SmtpClient was occupying.

The following code example demonstrates sending an e-mail message asynchronously.

This language is not supported or no code example is available.

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role not supported), Windows Server 2003 SP2

.NET Framework

.NET Framework Client Profile

Reference

Other Resources