windows phone smtp mail not working

I'm using this below DLL to send the email from windows phone apps.

https://github.com/LightBuzz/SMTP-WinRT

but unfortunately

A certificate chain processed but terminated in a root certificate which is not trusted by the trust provider.

(Exception from HRESULT: 0x800B0109)

the error will come.

I have copied my certificate to Trusted root certification authorities using MMC too. when I use Gmail or outlook server, it working good.

Can anyone tell me why this happens?