How to resolve exception error in processing. The server response was: 5.7.3 STA...

How to resolve exception error in processing. The server response was: 5.7.3 STARTTLS is required to send mail [MR2P264CA0029.FRAP264.PROD.OUTLOOK.COM] while sending mail in C#?

I am getting below exception

 while sending mail in c#
 Error in processing. The server response was: 5.7.3 STARTTLS is required to send mail [MR2P264CA0029.FRAP264.PROD.OUTLOOK.COM]

What I have tried:

Code used to send mail updated as below:
web.config
<pre>  <add key="AdminEmail" value="[email protected]"/>
    <add key="SMTPServer" value="smtp.office365.com"/>
    <add key="SMTPServerPort" value="587"/>

MailMessage objMailMsg = new MailMessage();
               MailAddress fromAddress = new MailAddress(Common.FromMail);
               SmtpClient smtpClient = new SmtpClient();

               int port = Common.SMTPPort;
               string FileName = Common.EMAIL_FILE_PATH+objEmail.AttachedFileName;
               string IPaddr = Common.SMTPServer;
               objMailMsg.From = fromAddress;

               string[] Toarray = objEmail.ToEmailId.Split(',');
               foreach (string ToAddress in Toarray)
               {
                   if (ToAddress.Trim() != string.Empty)
                       objMailMsg.To.Add(ToAddress.Trim());
               }
               if (objEmail.CCEmailId != null && objEmail.CCEmailId.Length > 0)
               {
                   string[] Ccarray = objEmail.CCEmailId.Split(',');
                   foreach (string CcAddress in Ccarray)
                   {
                       if(CcAddress.Trim()!= string.Empty)
                           objMailMsg.CC.Add(CcAddress.Trim());
                   }
               }

               char[] chrDelim = new char[] { ',' };

               foreach (string strSubstr in FileName.Split(chrDelim))
               {
                   if (File.Exists(strSubstr))
                   {
                       Attachment objAttachment = new Attachment(strSubstr);
                       objMailMsg.Attachments.Add(objAttachment);
                   }
               }
               objMailMsg.Subject = objEmail.EmailSubject;
               objMailMsg.Body = objEmail.EmailBody;
               objMailMsg.IsBodyHtml = true;
               if (port == 0)
               {
                   port = 25;

               }
               Common.WriteDDSLog("Email Sent :-"+FileName);
               smtpClient.Port = port;
               smtpClient.Host = IPaddr;
               smtpClient.Send(objMailMsg);