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Unable to connect to Plesk mail server via an email client: TLS handshaking: SSL_accept() failed

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Leonid Gukhman
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Plesk for Linux kb: technical ABT: Group B

Applicable to:

  • Plesk for Linux

Symptoms

  • When connecting to a mail server over SSL via an email client (e.g. Outlook), connection fails:

    Your server does not support the connection encryption type you have specified. Try changing the encryption method.

  • One of the following error messages appears in the /var/log/maillog logfile:

    postfix/smtpd[25460]: warning: TLS library problem: error:140760FC:SSL routines:SSL23_GET_CLIENT_HELLO:unknown protocol:s23_srvr.c:640:

    dovecot: pop3-login: Disconnected (no auth attempts in 0 secs): user=<>, rip=203.0.113.2, lip=203.0.113.3, TLS handshaking: SSL_accept() failed: error:140760FC:SSL routines:SSL23_GET_CLIENT_HELLO:unknown protocol

Cause

TLSv1, SSLv3 or SSLv2 protocol support, which is required for old email clients/old software (e.g, WinHTTP-based applications on Windows 7) is disabled in Postfix/Dovecot configuration.

By default, SSL protocols SSLv2 and SSLv3 are disabled in Postfix/Dovecot configuration as these protocols are vulnerable to the POODLE attack.

Note: SSLv3 is removed from the core crypto libraries since RHEL 8. TLS 1.0 and 1.1 are disabled in all cryptographic policies in RHEL 9. See the documentation for details.

Resolution

The best practice is to use the latest available version of email clients that support connections over TLS 1.1 and 1.2.

For WinHTTP-based applications, refer to the Microsoft article.

If for some reason it is not possible, apply the following steps:

Note: This guide is intended for Plesk administrators. If you are a domain owner, please contact your service provider for assistance with email account configuration.

Enable the TLSv1 protocol in Postfix configuration

  1. Connect to the Plesk server via SSH.

  2. Enable support for TLSv1 protocol:

    # plesk sbin pci_compliance_resolver --disable postfix

    # plesk sbin sslmng --services postfix --protocols 'TLSv1 TLSv1.1 TLSv1.2 TLSv1.3'

    To find out whether TLSv1 version is enabled in Postfix or not, run the command:

    # egrep "smtpd_tls_mandatory_protocols|smtpd_tls_protocols" /etc/postfix/main.cf

  3. Restart Postfix:

    # service postfix restart

Enable the TLSv1 protocol in Dovecot configuration

  1. Connect to the Plesk server via SSH.

  2. Open the file /etc/dovecot/conf.d/11-plesk-security-ssl.conf in a text editor (for example, vi editor) and change the values of ssl_min_protocol and as follows:

    ssl_min_protocol=TLSv1

    To find out whether TLSv1 version is enabled in Dovecot or not, run the command:

    # egrep "ssl_min_protocol" /etc/dovecot/conf.d/11-plesk-security-ssl.conf

    Note: If TLSv1 is not enabled it will not be shown in the output.

  3. Restart Dovecot service:

    # service dovecot restart

Enable the TLSv1 protocol in Courier configuration

  1. Connect to the Plesk server via SSH.

  2. Open the files /etc/courier-imap/pop3d-ssl and /etc/courier-imap/imapd-ssl in a text editor (for example, vi editor) and change the values of TLS_PROTOCOL and TLS_STARTTLS_PROTOCOL as follows:

    # grep PROTOCOL /etc/courier-imap/pop3d-ssl | grep -v ^#
    TLS_PROTOCOL=TLSv1+
    TLS_STARTTLS_PROTOCOL=TLSv1+

    # grep PROTOCOL /etc/courier-imap/imapd-ssl | grep -v ^#
    TLS_PROTOCOL=TLSv1+
    TLS_STARTTLS_PROTOCOL=TLSv1+

  3. Restart Courier services:

    # service courier-imaps restart
    # service courier-pop3s restart
    # service courier-imapd restart
    # service courier-pop3d restart
    # service xinetd restart

If an email client still does not allow to connect, enable the SSLv2 and SSLv3 protocols in Postfix configuration:

Warning: Such configuration is not recommended due to security vulnerabilities. Apply it at your own risk!

Enable the SSLv2 and SSLv3 protocols in Postfix

  1. Connect to the Plesk server via SSH.

  2. Open the file /etc/postfix/main.cf in a text editor and set an empty value for "smtpd_tls_mandatory_protocols" and "smtpd_tls_protocols":

    # egrep "smtpd_tls_mandatory_protocols|smtpd_tls_protocols" /etc/postfix/main.cf
    smtpd_tls_mandatory_protocols =
    smtpd_tls_protocols =

  3. Restart Postfix:

    # service postfix restart

Comments

1 comment

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  • Avatar
    Virgil Turner

    Testing IMAP TLS/SSL using both https://testconnectivity.microsoft.com and latest version of Microsoft Outlook client fails to negotiate cryptographic connection with the following log:

    ### BEGIN LOG ENTRY ###
    imap-login: Disconnected: Connection Closed: SSL_accept() failed: error:0A000102:SSL routines::unsupported protocol (no auth attempts in 0 secs): user=<>, tip=52.109.8.10, lip=77.68.96.59, TLS handshaking: SSL_accept() failed: error:0A000102:SSL routines::unsupported protocol, session <sessionIDhere>
    ### END OF LOG ENTRY ###

    If I try to verify from a Linux machine using openssl on IMAP SSL port 993 then I get the expected LetsEncrypt certificates with expected CN names matching the domain, along wiht Dovecot welcome "* OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE LITERAL+ AUTH=PLAIN AUTH=LOGIN AUTH=DIGEST-MD5 AUTH=CRAM-MD5]"

    I have disabled OCSP Stapling as I read that this is not supported on Dovecot.
    When I run an SSL report at ssllabs.com I notice that the ONLY version of TLS supported by the certificate is TLS 1.3 (TLS 1.2 and below disabled, and SSL 3/2 both disabled.)
    The LetsEncrypt wildcard certificate is using an RSA 2048 bits key and SHA256withRSA signature. The TLS 1.3 ciphers supported are: TLS_AES_128_GCM_SHA256; TLS_AES_256_GCM_SHA384; TLS_CHACHA20_POLY1305_SHA256. 

    Any ideas for explanations/solutions?

    0
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