[exim] unable to get local issuer certificate cert

I'm using self signed certificate with my exim. Every time my exim in
sending email (outgoing email) it logs error like

SSL verify error: depth=1 error=unable to get local issuer
certificate cert=/C=US/O=DigiCert Inc/OU=www.digicert.com/CN=DigiCert
SHA2 High Assurance Server CA

Its the same error for every receiver and I belive error is about my
certificate not remote one. Emails get delivered without problems
though. How can I either fix it or disable logging of this error.