SmtpClient.SendAsync（）没有收到我发送的所有邮件

Question

I've just written some test code looking at the System.Net.Mail.SmtpClient sending identical messages to myself.

int numClients = 10;
List<SmtpClient> mailClients = new List<SmtpClient>();
for (int i = 0; i < numClients; i++) {
    mailClients.Add(new SmtpClient(smtpHost));
}

MailMessage msg = new MailMessage("myAddress@eg.com", "myAddress@eg.com", "test message", "" );
foreach (SmtpClient c in mailClients) {
    c.SendAsync(msg, null);
}

This is all fine and executes without any problems except that I only receive 'n - 1' messages. ie If I send 10 messages I only recieve 9 in my inbox. If I send 50 I only receive 49 etc.

Note: If I change the code to use a blocking Send then I will always receive the right number of messages. eg

foreach (SmtpClient c in mailClients) {
    c.Send(msg);
} 

Any ideas?

Answer 1

Here are a few observations that may help:

• Only create one SmtpClient.
• Create multiple messages instead.
• SmtpClient implements IDisposable. Wrap it in using .
• MailMessage also implements IDisposable.

I suspect you may be running into a bug/issue with multiple SmtpClient instances that all wrap the same SMTP server. Using a single instance may resolve the issue.

UPDATE

Per MSDN:

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.

http://msdn.microsoft.com/en-us/library/x5x13z6h.aspx

So given your situation, there is almost no benefit to using SendAsync over Send. Your loop is probably stomping on something since you do not wait for the previous SendAsync to complete.

Here are a few thoughts:

• SendAsync will perform almost the same as Send if you are sending a bunch of emails. Just use Send.
• If you need parallel sending, use a Producer/Consumer pattern. One (or more) producing threads dump stuff into a queue to send, and multiple consuming threads each use one SmtpClient to send messages. This pattern is amazingly simple to implement with a BlockingCollection. See the example in MSDN http://msdn.microsoft.com/en-us/library/dd267312.aspx
• If you use enough threads, your SMTP server will be the bottleneck. Be aware of when you are overloading it.