如何知道smtp服务器是否支持ssl？

Question

如何知道终端上的smtp服务器是否支持ssl？

Answer 1

You can connect to the mail server on the standard port, send a EHLO command and see if the response includes the STARTTLS capability. This tells you that the mail server will let you run an encrypted session over a standard SMTP connection. For example:

$ telnet localhost smtp
Trying 127.0.0.1...
Connected to localhost
220 localhost ESMTP
EHLO localhost
250-localhost
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-STARTTLS  <--- here it is
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
QUIT
221 2.0.0 Bye

Alternatively, you can try connecting to the default SMTP-over-SSL port of 465/tcp, and assume that the mail server supports SSL if you get a positive response and a successful SSL handshake followed by a SMTP greeting.