Apr 18 14:58:11 setzconote.dyndns.org stunnel[15619]: LOG4[3]: CERT: Pre-verification error: unable to get local issuer>

Apr 18 14:58:11 setzconote.dyndns.org stunnel[15619]: LOG4[3]: Rejected by CERT at depth=0: CN=*.guam.net

It seems to be a coincidence that doesn't seem to have anything to do with the kernel version.  smtp1.guam.net have broken their deployment by only installing the end-entity (leaf) certificate, without the required intermediate certificate.  It's a common mistake.

If you cannot make smtp1.guam.net administrators fix their email server configuration, you can manually specify the intermediate certificate in your CAfile.pem.  The enclosed file includes both the intermediate certificate and the root certificate, so that you can replace "CApath = /etc/ssl/certs" with "CAfile = /etc/stunnel/g2.pem".

Best regards,
    Mike