In this article I will explain with an example, how to send emails using GMAIL Account Credentials and also make use of threading to improve performance in ASP.Net using C#.
HTML Markup
The HTML Markup consists of:
TextBox – For capturing the values of Recipient Email address, Subject, Body, Gmail account email address, Gmail account password.
FileUpload – For attaching file to the email.
Button – For sending email.
The Button has been assigned with an OnClick event handler.
<table border="0" cellpadding="0" cellspacing="0">
        <td style="width: 80px">To:</td>
        <td><asp:TextBox runat="server" ID="txtTo"></asp:TextBox></td>
        <td><asp:TextBox runat="server" ID="txtSubject"></asp:TextBox></td>
        <td valign="top">Body:</td>
        <td><asp:TextBox runat="server" ID="txtBody" TextMode="MultiLine" Height="150" Width="200"></asp:TextBox></td>
        <td>File Attachment:</td>
        <td><asp:FileUpload runat="server" ID="fuAttachment" /></td>
        <td>Gmail Email:</td>
        <td><asp:TextBox runat="server" ID="txtEmail"></asp:TextBox></td>
        <td>Gmail Password:</td>
        <td><asp:TextBox runat="server" ID="txtPassword" TextMode="Password"></asp:TextBox></td>
        <td><asp:Button runat="server" ID="btnSend" Text="Send "OnClick="SendAsyncEmail" /></td>
Mail Server Settings in Web.Config file
The following Mail Server settings need to be saved in the Web.Config file.
        <smtp deliveryMethod="Network">
You will need to import the following namespaces.
using System.IO;
using System.Net;
using System.Net.Mail;
using System.Threading;
using System.Configuration;
using System.Net.Configuration;
Sending email asynchronously in background using Thread
MailMessage Class Properties
Following are the required properties of the MailMessage class.
From – Sender’s email address.
To – Recipient(s) Email Address.
CC – Carbon Copies. (If any)
BCC – Blind Carbon Copies. (If any)
Subject – Subject of the Email.
Body – Body of the Email.
IsBodyHtml – Specify whether body contains text or HTML mark up.
Attachments – Attachments. (If any)
ReplyTo – ReplyTo Email address.
SMTP Class Properties
Following are the properties of the SMTP class.
Host – SMTP Server URL. (Gmail:
EnableSsl – Specify whether your host accepts SSL Connections. (Gmail: True)
UseDefaultCredentials – Set to True in order to allow authentication based on the Credentials of the Account used to send emails.
Credentials – Valid login credentials for the SMTP server. (Gmail: email address and password)
Port – Port Number of the SMTP server. (Gmail: 587)
When the SendAsyncEmail Button is clicked, the Recipient email address (to), the Sender email address (from), Subject and Body values are fetched from their respective fields and are passed as parameter to SendEmail function.
For sending email asynchronously in background, a new Thread object is defined and inside which the SendEmail function is called using delegate in C# and Sub in VB.Net.
Inside the SendEmail function, all these parameters are set into an object of the MailMessage class.
If the ASP.Net Multiple FileUpload control has attachment then, the attachment is added to the Attachments List of the MailMessage Object.
Then, object of the SmtpClient class is created and the settings of the Mail Server such has Host, Port and EnableSsl are fetched from the mailSettings section of the Web.Config file and are set in respective properties of the SmtpClient class object.
And Username and Password values are passed as parameter to NetworkCredential class from their respective fields.
Note: It is necessary to use the sender’s email address credentials while defining the Gmail SMTP Server Credentials as Gmail the sender’s email address must be equal to Gmail Username specified in credentials.
Finally, the email is being sent and success the message is displayed in JavaScript Alert Message Box using RegisterStartupScript method.
protected void SendAsyncEmail(object sender, EventArgs e)
    string to = txtTo.Text;
    string from = txtEmail.Text;
    string password = txtPassword.Text;
    string subject = txtSubject.Text;
    string body = txtBody.Text;
    HttpPostedFile postedFile = fuAttachment.PostedFile;
    Thread email = new Thread(delegate ()
        SendEmail(to, from, password, subject, body, postedFile);
    email.IsBackground = true;
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Email sent.');", true);
private void SendEmail(string to, string from, string password, string subject, string body, HttpPostedFile postedFile)
    SmtpSection smtpSection = (SmtpSection)ConfigurationManager.GetSection("");
    using (MailMessage mm = new MailMessage(from, to))
        mm.Subject = subject;
        mm.Body = body;
        if (postedFile.ContentLength > 0)
            string fileName = Path.GetFileName(postedFile.FileName);
            mm.Attachments.Add(new Attachment(postedFile.InputStream, fileName));
        mm.IsBodyHtml = false;
        using (SmtpClient smtp = new SmtpClient())
            smtp.Host = smtpSection.Network.Host;
            smtp.EnableSsl = smtpSection.Network.EnableSsl;
            NetworkCredential networkCred = new NetworkCredential(from, password);
            smtp.UseDefaultCredentials = smtpSection.Network.DefaultCredentials;
            smtp.Credentials = networkCred;
            smtp.Port = smtpSection.Network.Port;
Possible Errors
The following error occurs when you try to send email using Gmail credentials in your application.
The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at
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Send Email using GMAIL in ASP.Net using C#