SmtpClient does not close session after sending message

Question

I have a function that sends email using asp.net built in mail framework. I've included it below.

public void SendMessage()
{
    var message = new MailMessage();
    var client = new SmtpClient();

    // Get the Message Envelope Details
    this.LoadMessageDetailsFromFile();

    // Process rules (if any): Rules engine not implemented yet!
    this.SetConfiguration(message);

    // Formats the message body template using XSLT
    this.FormatMessageBody(message);

    // Adds the attachments 
    this.AddAttachments(message);


    // Send the mail
    client.Timeout = 999999999;
    client.Send(message);

    //Clean up attachments
    foreach (var attachment in message.Attachments)
    {
        attachment.Dispose();
    }
}

Now at no point is the total message size greater than 10MB, however exchange is producing an error saying that 20MB is the max size for each session. Does .net somehow batch send messages to SMTP? Why would it be producing this error, even when each time I send an email, I create a new SMtpClient object?

Exact Exchange Error: Session size exceeds fixed maximum session size

It seems the underlining connection is reused, even if you create new instances of SMTPClient.

Anyone know a workaround, the following does not work:

Smtp client = new SmtpClient("hostname");
client.ServicePoint.MaxIdleTime = 0;
client.ServicePoint.ConnectionLimit = 1;

Thanks in advance

The host for the SMTP server setting is the Exchange server itself? — o.k.w
I would suggest you look into the Exchange log (if possible) to check if the loop actually is considered as one session from exchange's perspective. — o.k.w
thanks, this function is part of a class, and I instantiate a new object from this class, during the main loop. Surely this means that the smtp client object is destroyed each time the object goes out of scope? — JL.
Side note: your cleanup loop can be replaced with message.Dispose();, it should take all the attachments with it. There's a note inside the documentation that says "Always call Dispose before you release your last reference to the MailMessage." — Rory

Rory Rory · Accepted Answer · 2010-01-07T08:08:32

Attachments on emails get encoded using base64, which could make them considerably larger depending on the content of the file. From the Wikipedia article on base64:

Thus, the actual length of MIME-compliant base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be a lot higher because of the overhead of the headers. Very roughly, the final size of base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers). In other words, you can approximate the size of the decoded data with this formula: bytes = (string_length(encoded_string) - 814) / 1.37. Please note that these formulas should not be used to calculate the exact memory size to hold encoded data since they are very rough and produce errors in both directions

SmtpClient does not close session after sending message

2 Answers