This seems to be pretty straight forward. I need to send email from some ASP.NET applications. I need to do this consistently without strange errors and without CPU utilization going through the roof. I'm not talking about mass emailing, just occasional emails.
System.Net.Mail appears to be horribly broken. The SmtpClient does not issue the Quit command (it may be because Microsoft(R) is not interested in following specifications), therefore a connection is left open. Therefore, if someone tries to email before that connection finally closes, you can get errors from the SMTP Server about too many connections open. This is a bug that Microsoft(R) is completely uninterested in fixing. See here:
http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=146711
Also, if you look around some suggest to use this code to solve this problem:
smtpClient.ServicePoint.MaxIdleTime = 1;
smtpClient.ServicePoint.ConnectionLimit = 1;
Ok, yes that does "solve" the problem of connections being left open. However, this consistently, try it on a server if you like, causes the CPU on which the process (in this case w3wp.exe) is running to jump and remain at 100% until your application pool is recycled. For whatever reason, the thread that runs mscorwks.dll!CreateApplicationContext is the culprit.
This has the very nice side effect that if you are running on a web host that frowns on sustained 100% CPU usage, you will get your application pool disabled. So this is not as trivial as some suggest.
So my question is what to do? What I need to do is so simple; however getting those "too many connections open" errors is not acceptable and nor is the 100% CPU usage. I don't want to purchase a third party component, not because I'm cheap, but I buy enough components and MSDN subscription that it seems crazy to have to shell out $100-$300 for simple SMTP functionality.
I read that setting the MaxIdleTime higher can help but I'm skeptical of that. I don't want to risk my app pool being disabled just because Microsoft doesn't want to follow the SMTP specification.
Edit: I looked at quiksoft.com components, however it does not support SMTP authentication and it costs $500. There's got to be a solution to this problem.
I faced the same CPU utilization problem with the settings described. I ended up opening a ticket with Microsoft to determine the cause of the problem. The CPU utilization problem lies in the ServicePoint class. Internally in the ServicePoint class, there is a timer that runs every (MaxIdleTime/2) milliseconds. See the problem? By changing the MaxIdleTime value to 2, the CPU utilization will drop down to normal levels.
In .NET 4.0, SmtpClient is now disposable. The SMTP QUIT command is issued upon disposal such as when used in a using block.
I've always used Quiksoft's EasyMail .NET components with no issues at all.
Product home page: http://www.quiksoft.com/emdotnet/
They also have a free version of the component if you only need to send out e-mails:
We have used hMailserver with great success. The configuration can take a while to get used to but it has been a great -- and free -- mail server product.
If you want to roll your own (which I did years ago when I was having a bear of a time with CDONTS), you can start with the code below and customize to your heart's content. It uses the TcpClient to create a TCP connection directly to the mailserver. Not that I'd recommend this when there are so many established and debugged solutions, but I found this very useful for debugging and determining where the problem was with the prefab MS mail components.
private void Send_Email()
{
#region Email Sending Function
string strMail = "";
try
{
// See RFC821 http://www.faqs.org/rfcs/rfc821.html for more specs
// TcpClient is an abstraction of a TCP Socket connection
TcpClient myTCP = new TcpClient();
// Connect to the mail server's port 25
myTCP.Connect(mailserver, 25);
// Open a network stream which sends data to/from the TcpClient's socket
System.Net.Sockets.NetworkStream ns = myTCP.GetStream();
// The data to send to the mail server, basically a raw SMTP mail message
strMail = "HELO\n";
strMail += "MAIL FROM:from@address.com\n";
strMail += "RCPT TO:" + recipient + "\n";
strMail += "DATA\n";
strMail += "Subject: mySubject\n";
strMail += "To:" + recipient + "\n";
strMail += "From: \"From Real Name\" <from@address.com>\n";
strMail += "\n";
strMail += " ---------------------------------------\n";
strMail += "Name: " + txtName.Text + "\n";
strMail += "Address1: " + txtAddress1.Text + "\n";
strMail += "Address2: " + txtAddress2.Text + "\n";
strMail += "City: " + txtCity.Text + "\n";
strMail += "State: " + txtState.Text + "\n";
strMail += "Zip: " + txtZip.Text + "\n";
strMail += "Email: " + txtEmail.Text + "\n";
strMail += "Dealer: " + txtDealer.Text + "\n";
strMail += " ---------------------------------------\n";
strMail += "THIS IS AN AUTOMATED EMAIL SYSTEM. DO NOT REPLY TO THIS ADDRESS.\n";
strMail += "\n.\n";
// Defines encoding of string into Bytes (network stream needs
// an array of bytes to send -- can't send strings)
ASCIIEncoding AE = new ASCIIEncoding();
byte[] ByteArray = AE.GetBytes(strMail);
// send the byte-encoded string to the networkstream -> mail server:25
ns.Write(ByteArray, 0, ByteArray.Length);
//ns.Read(ByteArray, 0, ByteArray.Length);
//lblStatus.Text = ByteArray.ToString();
// close the network stream
ns.Close();
// close the TCP connection
myTCP.Close();
}
catch(Exception ex)
{
throw new Exception("Couldn't send email: <p>" + ex.Message);
}
#endregion
}
While I haven't had any specific problems with System.Net.Mail so far, you can always use the older System.Web.Mail API which is a wrapper for CDOSYS.
I've used Quicksoft in the past and have no complaints. Another thing you can try is switching the SMTP configuration to use a pickup folder instead of sending using the network which should get around the "it doesn't send QUIT" issue.
I send most of my mail using a Sproc. I can even attach a file.
CREATE PROCEDURE [dbo].[sendMail_With_CDOMessage]
@to VARCHAR(64),
@CC VARCHAR(1024)='',
@BCC VARCHAR(1024)='',
@subject VARCHAR(500)='',
@body VARCHAR(8000)='' ,
@from VARCHAR(64),
@filename VARCHAR(255)='',
@priority INT = 0
AS
BEGIN
SET NOCOUNT ON
DECLARE
@handle INT,
@return INT,
@s VARCHAR(64),
@sc VARCHAR(1024),
@up CHAR(27),
@server VARCHAR(255)
SET @s = '"http://schemas.microsoft.com/cdo/configuration/'
SELECT
@s = 'Configuration.Fields(' + @s,
@up = 'Configuration.Fields.Update',
@server = 'smtp.yourdomain.com'
EXEC @return = sp_OACreate 'CDO.Message', @handle OUT
SET @sc = @s + 'sendusing").Value'
EXEC @return = sp_OASetProperty @handle, @sc, '2'
SET @sc = @s + 'smtpserver").Value'
EXEC @return = sp_OASetProperty @handle, @sc, @server
EXEC @return = sp_OAMethod @handle, @up, NULL
EXEC @return = sp_OASetProperty @handle, 'To', @to
EXEC @return = sp_OASetProperty @handle, 'CC', @CC
EXEC @return = sp_OASetProperty @handle, 'BCC', @BCC
EXEC @return = sp_OASetProperty @handle, 'From', @from
EXEC @return = sp_OASetProperty @handle, 'Subject', @subject
EXEC @return = sp_OASetProperty @handle, 'HTMLBody', @body
EXEC @return = sp_OASetProperty @handle, 'Priority', 'cdoHigh'
IF @filename IS NOT NULL
EXEC @return = sp_OAMethod @handle, 'AddAttachment', NULL, @filename
EXEC @return = sp_OAMethod @handle, 'Send', NULL
IF @return 0
BEGIN
PRINT 'Mail failed.'
IF @from IS NULL
PRINT 'From address undefined.'
ELSE
PRINT 'Check that server is valid.'
END
ELSE
PRINT 'Mail sent.'
EXEC @return = sp_OADestroy @handle
END
来源：https://stackoverflow.com/questions/930236/net-best-method-to-send-email-system-net-mail-has-issues