In this article I will explain with an example, how to configure GMAIL SMTP settings in ASP.Net using C# and VB.Net.
The SMTP Mail settings are stored in the mailSettings sub-section of the system.net section of the Web.Config file.
The SmtpSection class of the System.Net.Configuration namespace is used to read the SMTP Mail settings from Web.Config file in ASP.Net using C# and VB.Net.
 
 
HTML Markup
The HTML Markup has a form with some TextBoxes for Recipient Email address, Subject, Body and a Button.
The Button has been assigned OnClick event handler.
<table border="0" cellpadding="0" cellspacing="0">
    <tr>
        <td>To:</td>
        <td><asp:TextBox ID="txtTo" runat="server" /></td>
    </tr>
    <tr>
        <td>Subject:</td>
        <td><asp:TextBox ID="txtSubject" runat="server" /></td>
    </tr>
    <tr>
        <td valign = "top">Body:</td>
        <td><asp:TextBox ID="txtBody" runat="server" TextMode="MultiLine" Height="150" Width="200" /></td>
    </tr>
    <tr>
        <td></td>
        <td><asp:Button ID="btnSend" Text="Send" runat="server" OnClick = "SendEmail" /></td>
    </tr>
</table>
 
 
Mail Server Settings in Web.Config file
Following are the Mail Server settings in the Web.Config file.
<system.net>
    <mailSettings>
        <smtp deliveryMethod="Network" from="sender@gmail.com">
            <network
                host="smtp.gmail.com"
                port="587"
                enableSsl="true"
                userName="sender@gmail.com"
                password="SenderGmailPassword"
                defaultCredentials="true"/>
        </smtp>
    </mailSettings>
</system.net>
 
 

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: smtp.gmail.com)
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)
 
 
Namespaces
You will need to import the following namespaces.
C#
using System.Net;
using System.Net.Mail;
using System.Configuration;
using System.Net.Configuration;
 
VB.Net
Imports System.Net
Imports System.Net.Mail
Imports System.Configuration
Imports System.Net.Configuration
 
 
Configuring GMAIL SMTP settings in ASP.Net
When the Send Button is clicked, the Sender’s email address (from) is fetched from the SmtpSection of the Web.Config file, the Subject and Body are fetched from their respective TextBoxes and all these values are set into an object of the MailMessage class.
Then an object of the SmtpClient class is created and the settings of the Mail Server such has Host, Port, DefaultCredentials, EnableSsl, Username and Password are fetched from the mailSettings section of the Web.Config file and attached to the SmtpClient class object.
Finally, a Success message is displayed using JavaScript Alert Message Box.
C#
protected void SendEmail(object sender, EventArgs e)
{
    SmtpSection smtpSection = (SmtpSection)ConfigurationManager.GetSection("system.net/mailSettings/smtp");
    using (MailMessage mm = new MailMessage(smtpSection.From, txtTo.Text.Trim()))
    {
        mm.Subject = txtSubject.Text.Trim();
        mm.Body = txtBody.Text.Trim();
        mm.IsBodyHtml = false;
        SmtpClient smtp = new SmtpClient();
        smtp.Host = smtpSection.Network.Host;
        smtp.EnableSsl = smtpSection.Network.EnableSsl;
        NetworkCredential networkCred = new NetworkCredential(smtpSection.Network.UserName, smtpSection.Network.Password);
        smtp.UseDefaultCredentials = smtpSection.Network.DefaultCredentials;
        smtp.Credentials = networkCred;
        smtp.Port = smtpSection.Network.Port;
        smtp.Send(mm);
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Email sent.');", true);
    }
}
 
VB.Net
Protected Sub SendEmail(sender As Object, e As System.EventArgs)
    Dim smtpSection As SmtpSection = CType(ConfigurationManager.GetSection("system.net/mailSettings/smtp"), SmtpSection)
    Using mm As MailMessage = New MailMessage(smtpSection.From, txtTo.Text.Trim())
        mm.Subject = txtSubject.Text.Trim()
        mm.Body = txtBody.Text.Trim()
        mm.IsBodyHtml = False
        Dim smtp As SmtpClient = New SmtpClient
        smtp.Host = smtpSection.Network.Host
        smtp.EnableSsl = smtpSection.Network.EnableSsl
        Dim networkCred As NetworkCredential = New NetworkCredential(smtpSection.Network.UserName, smtpSection.Network.Password)
        smtp.UseDefaultCredentials = smtpSection.Network.DefaultCredentials
        smtp.Credentials = networkCred
        smtp.Port = smtpSection.Network.Port
        smtp.Send(mm)
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('Email sent.');", True)
    End Using
End Sub
 
 
Possible Errors
Errors while sending Email using Gmail
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
Solution
 
 
Screenshots
Email Form
Configure GMAIL SMTP settings in ASP.Net
 
Received Email
Configure GMAIL SMTP settings in ASP.Net
 
 
Downloads