Hello Stunnel maintainers,

I try to use real certificates of my web server for stunnel. I combine private key, my site's cert, and ca-bundle into a pem file, and it works fine when the client doesn't verify any certificate. Then I specify CAfile which is the ca bundle file from my registrar, at client side and turn on verification and always get errors below, whatever level 2 or 3:
Sep 15 14:53:28 y400 stunnel[11666]: LOG5[11]: Service [http-proxy3] connected remote server from 192.168.1.104:45746
Sep 15 14:53:28 y400 stunnel[11666]: LOG4[11]: CERT: Pre-verification error: unable to get issuer certificate
Sep 15 14:53:28 y400 stunnel[11666]: LOG4[11]: Rejected by CERT at depth=2:

However, level 4 works. I want to prevent man-in-middle-attack, so can level 4 achieve that regarding to my current setup?
Both server and client side use stunnel 5.17 which are fairly recent.