How to send an email with attachments using SmtpClient.SendAsync?

Don't use "using" here. You are destroying the memory stream immediately after calling SendAsync, e.g. probably before SMTP gets to read it (since it's async). Destroy your stream in the callback.

I have tried your function and it works even for email with in memory attachments. But here are some remarks:

• What type of attachments did you try to send ? Exe ?
• Are both sender and receiver on the same email server ?
• You should “catch” exception and not just swallow it, than you will get more info about your problem.

• What does the exception says ?

• Does it work whan you use Send instead of SendAsync ? You are using 'using' clause and closing Stream before email is sent.

Here is good text about this topic:

Sending Mail in .NET 2.0

An extension to the Solution supplied in the original question also correctly cleans up an attachments that may also require disposal.

    public event EventHandler EmailSendCancelled = delegate { };    public event EventHandler EmailSendFailure = delegate { };    public event EventHandler EmailSendSuccess = delegate { };    ...        MemoryStream mem = new MemoryStream();        try        {            thisReport.ExportToPdf(mem);            // Create a new attachment and put the PDF report into it.            mem.Seek(0, System.IO.SeekOrigin.Begin);            //Attachment att = new Attachment(mem, "MyOutputFileName.pdf", "application/pdf");            Attachment messageAttachment = new Attachment(mem, thisReportName, "application/pdf");            // Create a new message and attach the PDF report to it.            MailMessage message = new MailMessage();            message.Attachments.Add(messageAttachment);            // Specify sender and recipient options for the e-mail message.            message.From = new MailAddress(NOES.Properties.Settings.Default.FromEmailAddress, NOES.Properties.Settings.Default.FromEmailName);            message.To.Add(new MailAddress(toEmailAddress, NOES.Properties.Settings.Default.ToEmailName));            // Specify other e-mail options.            //mail.Subject = thisReport.ExportOptions.Email.Subject;            message.Subject = subject;            message.Body = body;            // Send the e-mail message via the specified SMTP server.            SmtpClient smtp = new SmtpClient();            smtp.SendCompleted += SmtpSendCompleted;            smtp.SendAsync(message, message);        }        catch (Exception)        {            if (mem != null)            {                mem.Dispose();                mem.Close();            }            throw;        }    }    private void SmtpSendCompleted(object sender, AsyncCompletedEventArgs e)    {        var message = e.UserState as MailMessage;        if (message != null)        {            foreach (var attachment in message.Attachments)            {                if (attachment != null)                {                    attachment.Dispose();                }            }            message.Dispose();        }        if (e.Cancelled)            EmailSendCancelled?.Invoke(this, EventArgs.Empty);        else if (e.Error != null)        {            EmailSendFailure?.Invoke(this, EventArgs.Empty);            throw e.Error;        }        else            EmailSendSuccess?.Invoke(this, EventArgs.Empty);    }