我正在尝试使用 stunnel 加密电子邮件并将其发送到 Gmail 或 Outlook 帐户，我已激活 POP3/和 IMAP 客户端。但是当我运行该程序时，我收到以下错误消息：
我已经仔细检查了防火墙和发送，但仍然不明白这里的问题！有人知道我该如何解决这个问题吗？提前谢谢！ 更新
顺便说一下，我尝试了一个虚拟邮件，使用smtp4dev并且它起作用了，所以我猜测问题出在 stunnel 方面。这里是配置文件：
; Sample stunnel configuration file for Win32 by Michal Trojnara 2002-2017
; Some options used here may be inadequate for your particular configuration
; This sample file does *not* represent stunnel.conf defaults
; Please consult the manual for detailed description of available options
; **************************************************************************
; * Global options *
; **************************************************************************
; Debugging stuff (may be useful for troubleshooting)
;debug = info
;output = stunnel.log
; Enable FIPS 140-2 mode if needed for compliance
;fips = yes
; Microsoft CryptoAPI engine allows for authentication with private keys
; stored in the Windows certificate store
; Each section using this feature also needs the "engineId = capi" option
;engine = capi
; The pkcs11 engine allows for authentication with cryptographic
; keys isolated in a hardware or software token
; MODULE_PATH specifies the path to the pkcs11 module shared library,
; e.g. softhsm2.dll or opensc-pkcs11.so
; Each section using this feature also needs the "engineId = pkcs11" option
;engine = pkcs11
;engineCtrl = MODULE_PATH:softhsm2.dll
;engineCtrl = PIN:1234
; **************************************************************************
; * Service defaults may also be specified in individual service sections *
; **************************************************************************
; Enable support for the insecure SSLv3 protocol
;options = -NO_SSLv3
; These options provide additional security at some performance degradation
;options = SINGLE_ECDH_USE
;options = SINGLE_DH_USE
; **************************************************************************
; * Include all configuration file fragments from the specified folder *
; **************************************************************************
;include = conf.d
; **************************************************************************
; * Service definitions (at least one service has to be defined) *
; **************************************************************************
; ***************************************** Example TLS client mode services
[gmail-pop3]
client = yes
accept = 111
connect = pop.gmail.com:995
verifyChain = yes
CAfile = ca-certs.pem
checkHost = pop.gmail.com
OCSPaia = yes
[gmail-imap]
client = yes
accept = 143
connect = imap.gmail.com:993
verifyChain = yes
CAfile = ca-certs.pem
checkHost = imap.gmail.com
OCSPaia = yes
[gmail-smtp]
client = yes
accept = 127.0.0.1:25
connect = smtp.gmail.com:465
verifyChain = yes
CAfile = ca-certs.pem
checkHost = smtp.gmail.com
OCSPaia = yes
[hotmail-pop3]
client = yes
accept = 110
connect = pop-mail.outlook.com:995
verifyChain = yes
CAfile = ca-certs.pem
checkHost = pop-mail.outlook.com
OCSPaia = yes
[hotmail-smtp]
client = yes
accept = 26
connect = smtp-mail.outlook.com:587
verifyChain = yes
CAfile = ca-certs.pem
checkHost = smtp-mail.outlook.com
OCSPaia = yes
protocol = smtp
; Encrypted HTTP proxy authenticated with a client certificate
; located in the Windows certificate store
;[example-proxy]
;client = yes
;accept = 127.0.0.1:8080
;connect = example.com:8443
;engineId = capi
; Encrypted HTTP proxy authenticated with a client certificate
; located in a cryptographic token
;[example-pkcs11]
;client = yes
;accept = 127.0.0.1:8080
;connect = example.com:8443
;engineId = pkcs11
;cert = pkcs11:token=MyToken;object=MyCert
;key = pkcs11:token=MyToken;object=MyKey
; ***************************************** Example TLS server mode services
;[pop3s]
;accept = 995
;connect = 110
;cert = stunnel.pem
;[imaps]
;accept = 993
;connect = 143
;cert = stunnel.pem
;[ssmtp]
;accept = 465
;connect = 25
;cert = stunnel.pem
; TLS front-end to a web server
;[https]
;accept = 443
;connect = 80
;cert = stunnel.pem
; "TIMEOUTclose = 0" is a workaround for a design flaw in Microsoft SChannel
; Microsoft implementations do not use TLS close-notify alert and thus they
; are vulnerable to truncation attacks
;TIMEOUTclose = 0
; Remote cmd.exe protected with PSK-authenticated TLS
; Create "secrets.txt" containing IDENTITY:KEY pairs
;[cmd]
;accept = 1337
;exec = c:\windows\system32\cmd.exe
;execArgs = cmd.exe
;ciphers = PSK
;PSKsecrets = secrets.txt
; vim:ft=dosini