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

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Kuzma Ivanov
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Applicable to:

  • Plesk for Linux

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 uses port 993:

    1. connect to a mail server using openssl:

      # openssl s_client -showcerts -connect mail.example.com:993

    2. Check output and make sure that a valid certificate is shown:

      CONFIG_TEXT: 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

    3. Make sure that you received IMAP server response:

      CONFIG_TEXT: * 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 uses port 995:

    1. Connect to a mail server using openssl:

      # openssl s_client -showcerts -connect mail.example.com:995

    2. Check output and make sure that a valid certificate is shown:

      CONFIG_TEXT: 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

    3. Make sure that you received POP3 server response:

      CONFIG_TEXT: +OK Hello there. <1793.1385684315@localhost.localdomain>

       

  • SMTP via SSL uses port 465:

    1. Connect to a mail server using openssl:

      # openssl s_client -showcerts -connect mail.example.com:465

    2. Check output and make sure that a valid certificate is shown:

      CONFIG_TEXT: 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

    3. Make sure that you received SMTP server response:

      CONFIG_TEXT: 220 mail.example.com ESMTP Postfix

       

  • SMTP via TLS/StartTLS uses port 25 or 587 (The submission port should be enabled in Plesk):

    1. Connect to a mail server using openssl:

      # openssl s_client -starttls smtp -showcerts -connect mail.example.com:25

    2. Check output and make sure that a valid certificate is shown:

      CONFIG_TEXT: 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

    3. Make sure that you received SMTP server response:

      CONFIG_TEXT: 250 DSN

Comments

6 comments

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  • Avatar
    Henri Pelkonen

    I have similar problem, as OVH Let's encrypt SSL won't work, and mail certifies by domain always resort to ovh address.

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  • Avatar
    Lev Iurev

    @Henri, could you please describe the issue in more details?

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  • Avatar
    Henri Pelkonen (Edited )

    @Lev Iurev

    Hi,

    For starters I must say I'm very new to this and this is my first server setup with Plesk and mail server.

    Would you recommend 1 serverwide certificate to secure multiple domains or every domain with own sertificate as my vps is with shared IP?

    The problem is that I can't make sertificate now for my OVH vps cause number of *.ovh.net certs are restricted by Let's Encrypt. So should I get a own domain for vps and plesk server and get sertificate to that or could and should I get separate domain name sertificates to work with each domain? As for now the domains seems to show vps plesk self signed certificate even though they are set to Let's Encrypt via Plesk panel.

    Is there a way to force certificates to domain and bypass the plesk certificate and check them after they are set that tey truly are correct? Do I just need some refresh? I have use something like for now:

    true | openssl s_client -connect YOUR-DOMAIN.COM:995 | openssl x509 -noout -text | grep DNS:
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  • Avatar
    Artyom Baranov


    @Henri Pelkonen,

    >Would you recommend 1 serverwide certificate to secure multiple domains or every domain with own sertificate as my vps is with shared IP?

    I would use free Let's Encrypt certificates to secure my domains.

    >Is there a way to force certificates to domain and bypass the plesk certificate and check them after they are set that tey truly are correct?

    Make sure that SSL/TLS support is enabled and Let's Encrypt certificate is selected in Domains > example.com > Hosting Settings > Security

    Afterwards, use the following command to check which cert is used for a domain (where example.com is the domain name):

    # openssl s_client -showcerts -servername example.com -connect example.com:443

     

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  • Avatar
    Alaa Mansour

    is it normal that when i request:
    openssl s_client -showcerts -connect mail.example.com:995

    i recieve my server certificate:
    s:/CN=my.server.com
    ???

    the first address is for a client, and they use my.server.com as an address for using IMAP and SMTP, not there domain name

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  • Avatar
    Nikita Nikushkin

    Hello @Alaa Mansour,

    This report depends on a certificate selected for securing mail in Plesk:

    How to secure a Plesk mail server with an SSL certificate (Let's Encrypt / other certificate authorities)?

    You have selected a certificate issued for the server hostname for the "Certificate for securing mail" at the "Plesk > Tools & Settings > SSL/TLS certificates" page, thus, you receive the next output:

    # openssl s_client -showcerts -connect mail.example.com:995

    s:/CN=my.server.com

    If you select certificate issued for a website, e.g. example.com, for securing mail, the output will be the following:

    # openssl s_client -showcerts -connect mail.example.com:995

    s:/CN=www.example.com

    Keep in mind that an SSL certificate secures the entire mail server and all domains on it. Currently, it is not possible to secure domains in Plesk with a separate SSL certificate for the mail server

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