Knowledge Base

How to verify that SSL for IMAP/POP3/SMTP works and a proper SSL certificate is in use

Question

How to verify that SSL for IMAP/POP3/SMTP works and a proper certificate is installed?

Answer

Using online checkers

Check SSL using online tools:

Using a Linux server

Any Linux server can be used for these tests. If you do not have a Linux server, use the online checkers above.

To verify SSL, connect to any Linux server via SSH and use the instructions below:

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.

POP3 via SSL using port 995:

Connect to a mail server using openssl:

# openssl s_client -showcerts -connect mail.example.com:995 -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 POP3 server response:
```
+OK Hello there. [email protected]
```

SMTP via SSL using port 465:

Connect to a mail server using openssl:

# openssl s_client -showcerts -connect mail.example.com:465 -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:
```
220 mail.example.com ESMTP Postfix
```

SMTP via TLS/StartTLS using port 25 or 587

Note: To use port 587 the submission port should be enabled in Plesk:

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