openssl command smtp(ssl) 및 imap(ssl) 접속확인

To connect using the TLS protocol on port 587, use:

[darksharavim]openssl s_client -starttls smtp -connect smtp.mail.nate.com:587

smtp-ssl(465)

[darksharavim]openssl s_client -connect smtp.mail.nate.com:465

imap-ssl(993)

[darksharavim]openssl s_client -crlf -connect imap.nate.com:993

https://stackoverflow.com/questions/14959461/how-to-talk-to-imap-server-in-shell-via-openssl

How to talk to IMAP server in Shell via OpenSSL

인증서 유효기간 확인

## 로컬 https, smtps, pops, imaps 인증서 유효기간 확인시 참조
echo | openssl s_client -servername localhost -connect localhost:443 2>/dev/null | openssl x509 -noout -dates
echo | openssl s_client -servername localhost -connect localhost:465 2>/dev/null | openssl x509 -noout -dates
echo | openssl s_client -servername localhost -connect localhost:995 2>/dev/null | openssl x509 -noout -dates
echo | openssl s_client -servername localhost -connect localhost:993 2>/dev/null | openssl x509 -noout -dates

출처 : https://www.stevenrombauts.be/2018/12/test-smtp-with-telnet-or-openssl/