Answers:
您可以telnet用来连接到邮件服务器并使用POP3来检查您的凭据：
$ telnet pop.gmx.net 110
Trying 212.227.17.185...
Connected to pop.gmx.net.
Escape character is '^]'.
+OK POP server ready H migmx028 0MAbjW-1YwF4D0ml8-00BiVl
USER spamaccount80@gmx.de
+OK password required for user "spamaccount80@gmx.de"
PASS typeyourpassword
-ERR Error retrieving your GMX emails. Your connection is not encrypted. Enable SSL in your mail program. Instructions: https://ssl.gmx.net
Connection closed by foreign host.
好吧，这失败了，因为当今大多数邮件服务器都需要SSL / TLS加密的会话。因此，除了使用之外，telnet您还可以使用socat：
$ socat - OPENSSL:pop.gmx.net:995
+OK POP server ready H migmx113 0MC062-1Yzese0KO7-00AVNE
USER spamaccount80@gmx.de
+OK password required for user "spamaccount80@gmx.de"
PASS typeyourpassword
+OK mailbox "spamaccount80@gmx.de" has 13518 messages (191718918 octets) H migmx113
如果您输入了错误的密码，服务器可能会显示类似以下内容：
-ERR authentication failed
或者，socat您可能会openssl躺在周围：
$ openssl s_client -quiet -connect pop.gmx.net:995
depth=2 C = DE, O = Deutsche Telekom AG, OU = T-TeleSec Trust Center, CN = Deutsche Telekom Root CA 2
verify error:num=19:self signed certificate in certificate chain
verify return:0
+OK POP server ready H migmx108 0MWpjO-1YiwnK3ZfP-00XoK
telnet和openssl例子。
getmail也可以。