Thank you Brent,

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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?

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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...