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We are regularly sending out bulk emails (no spam, of course, just newsletters) for our customers, using Windows 2003, 2008 and 2008 R2 servers in Germany and the USA, and the number of mails we manage to send in one hour varies between 5.000 and 12.000, depending on whether we use a pickup folder, the local SMTP or a commercial SMTP server. We are using simple straightforward scripts with standard .NET libs or MailBee components.

Is that good? Average? Bad? Is there any obvious bottleneck (i.e. would it make sense to write a multi-threaded scripts or is it the connection to the mail server that limits the speed?)?

Olaf
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  • Are you saying that it takes your process 1 hour to generate 12.000 emails? Or are you saying that all generated emails are physically sent by your smtp server in an hour? – Shiv Kumar Feb 25 '11 at 11:46
  • I'm talking about the whole process, including the physical sending. – Olaf Feb 25 '11 at 13:33

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There are a few things that will limit the performance of this process outside of any code you've written.

  1. The Internet bandwidth on the server that is sending the emails.
  2. To where these emails are sent. That is your smtp server eventually has to connect and communicate with email recipient's email provider's server in order to send an email. Some servers are slow, some are busy and just take a long time to connect.

As regards using multiple threads to speed it up, Well, where is the slow down?

Email sending (and receiving) is mostly an I/O bound task. throwing threads at this problem may or many not improve performance. It all depends on if there is a bottleneck (in code) somewhere are are you just limited by the processing power or bandwidth of the server.

Check out this SO post I answered where I provided a multi-threaded solution that might help. Essentially it uses a parallel programming technique called "Data Parallel" where the workload is essentially broken up into multiple chunks and each chunk is then processed by a thread keeping the number of chunks the same as (or close to) the number of cores on the machine.

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Shiv Kumar
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  • Thanks for this. It doesn't really answer the question about what a good "email send performance" is, but I like the multi-threading approach you have linked to. – Olaf Feb 28 '11 at 07:48
  • @Olaf, well you didn't really answer my questions I asked in comments :). Also, the SmtpClient as an Asyn method, you mat want to try as well. I have an extensive post on my blog an HttpWebRequest and Async methods http://www.matlus.com/httpwebrequest-asynchronous-programming/ it should be simple to use the techniques present to use the SmtpClient in async mode. Note also that the article demostrates that multiple threads for I/O tasks don't really help. Your mileage may vary depending on what you consider "sent". – Shiv Kumar Feb 28 '11 at 08:02
  • @Shiv: Thanks again, I'll check the blog post. My - maybe naive - definition of "sent" is "delivered to the next machine, MTA, whatever", meaning "has left the server". The question should have been, maybe, "How do I measure the bulk mail performance on an IIS" followed by "What part of the email process chain has the best chance to be optimized?" – Olaf Feb 28 '11 at 16:16
  • @Olaf, ok so you can't control how fast emails are sent out from your server to the destination mailbox. Of course the bandwidth your sending server has plays a role and that could be a bottle neck because servers could use 500Mbps to 1Gbps (depending on how many emails are being sent) during that time the emails are being sent. But honestly, since the other side (the recipient's mail server) needs to be available as well you can't improve the performance all the way to the other party's mail server. – Shiv Kumar Feb 28 '11 at 19:49
  • If the emails are sitting in your smtp server's queue you know your C# process is fast enough. Maybe the server bandwidth is the bottle neck? You can confirm that the next time you send out bulk emails by looking at how much network bandwidth is being used. If it is 80% of the bandwidth you have from your ISP and/or the capacity of the NIC on your box, then it's quite likely bandwidth is the bottle neck. But there is no "standard" for "fast enough". If your "business" was sending bulk emails then your performance requirements would be very different and so will the hardware/bandwidth you use. – Shiv Kumar Feb 28 '11 at 19:51
  • @Shiv: good point, and yes, the c# process is certainly fast enough - the pickup folder fills quickly. What's weird is that the emails are sent at twice as fast from a Virtual Server in Germany than from a Root Server in St. Louis. That's why I was wondering in the first place if I'm missing an obvious bottle neck. However, the next step would be to check the bandwidth usage - thanks again for that hint (and also thanks for taking the time to discuss this in depth!!). – Olaf Mar 02 '11 at 11:13