Thank you Brent,
I found if I have Avast running with mail shield enabled I get this error in the Stunnel log window: SSL_connect: 140770FC: error:140770FC:SSL routines:SSL23_GET_SERVER_HELLO:unknown protocol Connection reset: 0 byte(s) sent to TLS, 0 byte(s) sent to socket. This results in no emails being abled to be sent by SMPT. If I disable Avast's mail shield the mail is sent with no errors. Is there a work around for to be able to use Stunnel without disabling Avast's mail shield?
POP3 Connection CERT: Pre-verification error: self signed certificate in certificate chain Rejected by CERT at depth=1: OU=generated by Avast Antivirus for SSL/TLS scanning, O=Avast Web/Mail Shield, CN=Avast Web/Mail Shield Root SSL_connect: 14090086: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed
If you need avast, sign your stunnel leaf with a well known root CA, and exchange certificates accordingly.
cox_mail : avast_dpi : stunnel_TLS_proxy : client
https://support.postbox-inc.com/hc/en-us/articles/204602300-Invalid-Security...