External POP3 Email sending error

External POP3 Email sending error

I am trying to set up external pop3 account.
My mail server requires incoming port 110, outgoing port 587.
After configuration, I can receive email but came across sending fail.
I'm receiving the following when I try to send email.  I realize it must have something to do with the certification of the domain I'm sending to but I don't know how to fix it. My mail server requires some certification (I always set up outgoing encription to AUTO)

Reason:sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target


Any help would be appreciated.

Hoonheui