vb.net - ASP.net Contact Form Sending an email error -

i need on 1 thing. have website , in website have contact page in users can contact admin page or other stuff.

the problem when try render page, gives me error saying

the smtp server requires secure connection or client not authenticated. server response was: 5.5.1 authentication required. learn more @ description: unhandled exception occurred during execution of current web request. please review stack trace more information error , originated in code.  exception details: system.net.mail.smtpexception: smtp server requires secure connection or client not authenticated. server response was: 5.5.1 authentication required. learn more @  source error:   line 21:         smtp.credentials = networkcred line 22:         smtp.port = 587 line 23:         smtp.send(mm) line 24:         lblmessage.text = "email sent sucessfully." line 25:     end sub   source file: c:\users\user\desktop\contactusform\vb.aspx.vb    line: 23  

i don't know wt do. vb behind code :

imports system.net imports system.net.mail partial class vb     inherits system.web.ui.page     protected sub btnsend_click(byval sender object, byval e eventargs)         dim mm new mailmessage("sender@gmail.com", "reveiver@gmail.com")         mm.subject = txtsubject.text         mm.body = "name: " & txtname.text & "<br /><br />email: " & txtemail.text & "<br />" & txtbody.text         if fileupload1.hasfile             dim filename string = system.io.path.getfilename(fileupload1.postedfile.filename)             mm.attachments.add(new attachment(fileupload1.postedfile.inputstream, filename))         end if         mm.isbodyhtml = true         dim smtp new smtpclient()         smtp.host = "smtp.gmail.com"         smtp.enablessl = true         dim networkcred new system.net.networkcredential()         networkcred.username = "sender@gmail.com"         networkcred.password = "senderpassword"         smtp.usedefaultcredentials = true         smtp.credentials = networkcred         smtp.port = 587         smtp.send(mm)         lblmessage.text = "email sent sucessfully."     end sub end class 

the error @ line 23 saying smtp.send(mm)

i in need. please can me.

change

smtp.usedefaultcredentials = false 

your don't want use default credentials, want use credentials created in code.

https://msdn.microsoft.com/en-us/library/system.net.mail.smtpclient.usedefaultcredentials(v=vs.110).aspx