Hi akhter,

I have made reference to below article to make below sample.

Send email using jQuery AJAX in ASP.Net

Make AJAX call using XmlHttpRequest (XHR) in ASP.Net

Refer below sample.

Web.Config

<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>

HTML

<input id="btnGetTime" type="button" value="Send Email" onclick="SendEmail()" />
<script type="text/javascript">
    function SendEmail() {
        var toEmail = "testEmail@aspforums.com";
        var subject = "testSubject";
        var body = "testBody";
        var request;
        if (window.XMLHttpRequest) {
            //New browsers.
            request = new XMLHttpRequest();
        }
        else if (window.ActiveXObject) {
            //Old IE Browsers.
            request = new ActiveXObject("Microsoft.XMLHTTP");
        }
        if (request != null) {
            var url = "Default.aspx/SendEmail";
            request.open("POST", url, false);
            var params = "{ toEmail: '" + toEmail + "', subject: '" + subject + "', body: '" + body + "' }";
            request.setRequestHeader("Content-Type", "application/json");
            request.onreadystatechange = function () {
                if (request.readyState == 4 && request.status == 200) {
                    alert(JSON.parse(request.responseText).d);
                }
            };
            request.send(params);
        }
    }
</script>

Namespaces

C#

using System.Net;
using System.Net.Mail;
using System.Web.Services;
using System.Configuration;
using System.Net.Configuration;

VB.Net

Imports System.Net
Imports System.Net.Mail
Imports System.Web.Services
Imports System.Configuration
Imports System.Net.Configuration

Code

C#

[WebMethod]
public static string SendEmail(string toEmail, string subject, string body)
{
    SmtpSection smtpSection = (SmtpSection)ConfigurationManager.GetSection("system.net/mailSettings/smtp");
    using (MailMessage mm = new MailMessage(smtpSection.From, toEmail))
    {
        mm.Subject = subject;
        mm.Body = body;
        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);
    }
    return "Email sent.";
}

VB.Net

<WebMethod>
Public Shared Function SendEmail(ByVal toEmail As String, ByVal subject As String, ByVal body As String) As String
    Dim smtpSection As SmtpSection = CType(ConfigurationManager.GetSection("system.net/mailSettings/smtp"), SmtpSection)
    Using mm As MailMessage = New MailMessage(smtpSection.From, toEmail)
        mm.Subject = subject
        mm.Body = body
        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)
    End Using
    Return "Email sent."
End Function