SmtpClient.SendAsync Method (MailMessage, Object)


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Sends the specified e-mail message to an SMTP server for delivery. This method does not block the calling thread and allows the caller to pass an object to the method that is invoked when the operation completes.

Namespace:   System.Net.Mail
Assembly:  System (in System.dll)

[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>]
member SendAsync : 
        message:MailMessage *
        userToken:Object -> unit


Type: System.Net.Mail.MailMessage

A MailMessage that contains the message to send.

Type: System.Object

A user-defined object that is passed to the method invoked when the asynchronous operation completes.

Exception Condition

message is null.


MailMessage.From is null.


This SmtpClient has a SendAsync call in progress.


There are no recipients specified in MailMessage.To, MailMessage.CC, and MailMessage.Bcc properties.


DeliveryMethod property is set to Network and Host is null.


DeliveryMethod property is set to Network and Host is equal to the empty string ("").


DeliveryMethod property is set to Network and Port is zero, a negative number, or greater than 65,535.


This object has been disposed.


The connection to the SMTP server failed.


Authentication failed.


The operation timed out.


EnableSsl is set to true but the DeliveryMethod property is set to SpecifiedPickupDirectory or PickupDirectoryFromIis.


EnableSsl is set to true, but the SMTP mail server did not advertise STARTTLS in the response to the EHLO command.


The message could not be delivered to one or more of the recipients in MailMessage.To, MailMessage.CC, or MailMessage.Bcc.

To receive notification when the e-mail has been sent or the operation has been canceled, add an event handler to the SendCompleted event. You can cancel a SendAsync operation by calling the SendAsyncCancel method.

After calling SendAsync, you must wait for the e-mail transmission to complete before attempting to send another e-mail message using Send or SendAsync.

Before calling this method, the Host and Port must be set through the configuration files by setting the relevant properties, or by passing this information into the SmtpClient(String, Int32) constructor.

If the SMTP host requires credentials, you must set them before calling this method. To specify credentials, use the UseDefaultCredentials or Credentials properties.

If you receive an SmtpException exception, check the StatusCode property to find the reason the operation failed. The SmtpException can also contain an inner exception that indicates the reason the operation failed.

When sending e-mail using SendAsync to multiple recipients, if the SMTP server accepts some recipients as valid and rejects others, a SmtpException is thrown with a NullReferenceException for the inner exception. If this occurs, SendAsync fails to send e-mail to any of the recipients.

Your application can detect a server certificate validation error by examining the Error property passed into the SendCompletedEventHandler delegate.

The Timeout property does not have any effect on a SendAsync call.

To send mail and block while it is transmitted to the SMTP server, use one of the Send methods.


If the EnableSsl property is set to true, and the SMTP mail server does not advertise STARTTLS in the response to the EHLO command, then a call to the Send or SendAsync methods will throw an SmtpException.

The following code example demonstrates calling this method.

No code example is currently available or this language may not be supported.


to connect to the SMTP server. Associated enumeration: Connect

.NET Framework
Available since 2.0
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