How to verify that SSL for IMAP/SMTP works and a proper SSL certificate is in use

Credit Robert Asibekov

IMAP via SSL using port 993

  • connect to a mail server using openssl:
openssl s_client -showcerts -connect mail.example.com:993 -servername mail.example.com
  • Check output and make sure that a valid certificate is shown:
Server certificate
subject=/OU=Domain Control Validated/OU=PositiveSSL/CN=mail.example.com
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=PositiveSSL CA 2
  • Make sure that you received IMAP server response:
* OK [CAPABILITY IMAP4rev1 UIDPLUS CHILDREN NAMESPACE THREAD=ORDEREDSUBJECT THREAD=REFERENCES SORT QUOTA AUTH=CRAM-MD5 AUTH=PLAIN IDLE ACL ACL2=UNION] Courier-IMAP ready. Copyright 1998-2004 Double Precision, Inc. See COPYING for distribution information.

SMTP via TLS/StartTLS using port 25 or 587

  • Connect to a mail server using openssl:
openssl s_client -starttls smtp -showcerts -connect mail.example.com:25 -servername mail.example.com
  • Check output and make sure that a valid certificate is shown:
Server certificate
subject=/OU=Domain Control Validated/OU=PositiveSSL/CN=mail.example.com
issuer=/C=GB/ST=Greater Manchester/L=Salford/O=COMODO CA Limited/CN=PositiveSSL CA 2
  • Make sure that you received SMTP server response:
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