Hello, 

I'm encountering an issue while using sTunnel with an Office365 account. 

sTunnel worked properly for a few months, while it gived an error with certificates since yesterday, whilst didn't change anything in the configuration. 

This is our configuration:

[pop3s]
client = yes
accept = 127.0.0.1:2001
connect = outlook.office365.com:995
CAfile = C:\Program Files (x86)\stunnel\config\ca-certs.pem
checkHost = outlook.office365.com
verifyChain = yes
OCSPaia = yes

This is what we get in the log:

2018.11.09 11:34:09 LOG7[main]: Found 1 ready file descriptor(s)
2018.11.09 11:34:09 LOG7[main]: FD=432 ifds=r-x ofds=---
2018.11.09 11:34:09 LOG7[main]: Service [pop3s] accepted (FD=672) from 127.0.0.1:49619
2018.11.09 11:34:09 LOG7[main]: Creating a new thread
2018.11.09 11:34:09 LOG7[main]: New thread created
2018.11.09 11:34:09 LOG7[30]: Service [pop3s] started
2018.11.09 11:34:09 LOG7[30]: Setting local socket options (FD=672)
2018.11.09 11:34:09 LOG7[30]: Option TCP_NODELAY set on local socket
2018.11.09 11:34:09 LOG5[30]: Service [pop3s] accepted connection from 127.0.0.1:49619
2018.11.09 11:34:09 LOG6[30]: failover: priority, starting at entry #0
2018.11.09 11:34:09 LOG6[30]: s_connect: connecting 40.101.9.178:995
2018.11.09 11:34:09 LOG7[30]: s_connect: s_poll_wait 40.101.9.178:995: waiting 10 seconds
2018.11.09 11:34:09 LOG5[30]: s_connect: connected 40.101.9.178:995
2018.11.09 11:34:09 LOG5[30]: Service [pop3s] connected remote server from 172.31.20.23:49620
2018.11.09 11:34:09 LOG7[30]: Setting remote socket options (FD=668)
2018.11.09 11:34:09 LOG7[30]: Option TCP_NODELAY set on remote socket
2018.11.09 11:34:09 LOG7[30]: Remote descriptor (FD=668) initialized
2018.11.09 11:34:09 LOG6[30]: SNI: sending servername: outlook.office365.com
2018.11.09 11:34:09 LOG6[30]: Peer certificate required
2018.11.09 11:34:09 LOG7[30]: TLS state (connect): before/connect initialization
2018.11.09 11:34:09 LOG7[30]: TLS state (connect): SSLv2/v3 write client hello A
2018.11.09 11:34:09 LOG7[30]: TLS state (connect): SSLv3 read server hello A
2018.11.09 11:34:09 LOG7[30]: Verification started at depth=2: C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
2018.11.09 11:34:09 LOG7[30]: CERT: Pre-verification succeeded
2018.11.09 11:34:09 LOG7[30]: OCSP: Ignoring root certificate
2018.11.09 11:34:09 LOG6[30]: Certificate accepted at depth=2: C=US, O=DigiCert Inc, OU=www.digicert.com, CN=DigiCert Global Root CA
2018.11.09 11:34:09 LOG7[30]: Verification started at depth=1: C=US, O=DigiCert Inc, CN=DigiCert Cloud Services CA-1
2018.11.09 11:34:09 LOG7[30]: CERT: Pre-verification succeeded
2018.11.09 11:34:09 LOG5[30]: OCSP: Connecting the AIA responder "http://ocsp.digicert.com"
2018.11.09 11:34:09 LOG6[30]: s_connect: connecting 93.184.220.29:80
2018.11.09 11:34:09 LOG7[30]: s_connect: s_poll_wait 93.184.220.29:80: waiting 10 seconds
2018.11.09 11:34:09 LOG5[30]: s_connect: connected 93.184.220.29:80
2018.11.09 11:34:09 LOG7[30]: OCSP: Connected ocsp.digicert.com:80
2018.11.09 11:34:09 LOG7[30]: OCSP: Response received
2018.11.09 11:34:09 LOG6[30]: OCSP: Status: good
2018.11.09 11:34:09 LOG6[30]: OCSP: This update: Nov 9 00:00:00 2018 GMT
2018.11.09 11:34:09 LOG6[30]: OCSP: Next update: Nov 16 00:00:00 2018 GMT
2018.11.09 11:34:09 LOG5[30]: OCSP: Certificate accepted
2018.11.09 11:34:09 LOG6[30]: Certificate accepted at depth=1: C=US, O=DigiCert Inc, CN=DigiCert Cloud Services CA-1
2018.11.09 11:34:09 LOG7[30]: Verification started at depth=0: C=US, ST=Washington, L=Redmond, O=Microsoft Corporation, CN=outlook.com
2018.11.09 11:34:09 LOG7[30]: CERT: Pre-verification succeeded
2018.11.09 11:34:09 LOG6[30]: CERT: Host name "outlook.office365.com" matched with "*.office365.com"
2018.11.09 11:34:09 LOG5[30]: OCSP: Connecting the AIA responder "http://ocspx.digicert.com"
2018.11.09 11:34:09 LOG6[30]: s_connect: connecting 93.184.220.29:80
2018.11.09 11:34:09 LOG7[30]: s_connect: s_poll_wait 93.184.220.29:80: waiting 10 seconds
2018.11.09 11:34:09 LOG5[30]: s_connect: connected 93.184.220.29:80
2018.11.09 11:34:09 LOG7[30]: OCSP: Connected ocspx.digicert.com:80
2018.11.09 11:34:09 LOG7[30]: OCSP: Response received
2018.11.09 11:34:09 LOG3[30]: OCSP: Responder error: 6: unauthorized
2018.11.09 11:34:09 LOG4[30]: Rejected by OCSP at depth=0: C=US, ST=Washington, L=Redmond, O=Microsoft Corporation, CN=outlook.com
2018.11.09 11:34:09 LOG7[30]: TLS alert (write): fatal: handshake failure
2018.11.09 11:34:09 LOG3[30]: SSL_connect: 14090086: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed
2018.11.09 11:34:09 LOG5[30]: Connection reset: 0 byte(s) sent to TLS, 0 byte(s) sent to socket
2018.11.09 11:34:09 LOG7[30]: Deallocating application specific data for session connect address
2018.11.09 11:34:09 LOG7[30]: Remote descriptor (FD=668) closed
2018.11.09 11:34:09 LOG7[30]: Local descriptor (FD=672) closed
2018.11.09 11:34:09 LOG7[30]: Service [pop3s] finished (0 left)

Can you please help me?

Thanks in advance!