I have installed the Postfix and enabled SSL/TLS, just tested, I can sent email from port 25, 578, but cannot sent email from port 465, the log is: May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept:SSLv3 write server hello A May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept:SSLv3 write certificate A May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept:SSLv3 write server done A May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept:SSLv3 flush data May 26 17:24:06 mail postfix/smtpd[28721]: SSL3 alert read:fatal:certificate unknown May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept:failed in SSLv3 read client certificate A May 26 17:24:06 mail postfix/smtpd[28721]: SSL_accept error from unknown[10.155.36.240]: 0 May 26 17:24:06 mail postfix/smtpd[28721]: warning: TLS library problem: 28721:error:14094416:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate unknown:s3_pkt.c:1197:SSL alert number 46: May 26 17:24:06 mail postfix/smtpd[28721]: lost connection after CONNECT from unknown[10.155.36.240] May 26 17:24:06 mail postfix/smtpd[28721]: disconnect from unknown[10.155.36.240] My email server is: 10.155.34.117, and email client is: 10.155.36.240, the client error is: Could not connect to SMTP host: 10.155.34.117, port: 465. My Master.cf: smtps inet n - n - - smtpd -o smtpd_tls_wrappermode=yes My main.cf: smtpd_use_tls = yes smtpd_tls_auth_only = no smtpd_tls_key_file = /etc/pki/myca/mail.key smtpd_tls_cert_file = /etc/pki/myca/mail.crt smtpd_tls_CAfile = /etc/pki/myca/cacert_new.pem smtpd_tls_loglevel = 2 smtpd_tls_received_header = yes smtpd_tls_session_cache_timeout = 3600s smtpd_tls_session_cache_database = btree:/etc/postfix/smtpd_scache Seems it's my certificate issue, but I have tried to grant the file many times...I have no idea on this, please help!