Cannot connect to IMAP server using MS Outlook: Socket error: 10061, Error Number: 0x800ccc0e

Symptoms

• IMAP connection to a Plesk server using MS Outlook does not work after Debian dist-upgrade:

  The connection to the server has failed. Account: ‘your_account’, Server:’your_SMTP_server’, Protocol: SMTP, Port: 25, Secure (SSL): NO, Socket error: 10061, Error Number: 0x800ccc0e

• The following error can be found in the file /var/log/syslog:

  plesk.example.com dovecot: imap-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:14094416:SSL routines:SSL3_READ_BYTES:sslv3 alert certificate unknown: SSL alert number 46, session=<sKc5E7VZXlVtWyft>

• SSH connection or access to Plesk administrative panel is temporarily blocked.

• Fail2Ban is enabled and IP address of your workstation can be found in /var/log/fail2ban.log :

  fail2ban.filter [2202]: INFO [plesk-dovecot] Found 203.0.113.2
  fail2ban.filter [2202]: INFO [plesk-dovecot] Found 203.0.113.2
  fail2ban.filter [2202]: INFO [plesk-dovecot] Found 203.0.113.2
  fail2ban.filter [2202]: INFO [plesk-dovecot] Found 203.0.113.2
  fail2ban.actions [2202]: NOTICE [plesk-dovecot] Ban 203.0.113.2
  fail2ban.filter [2202]: INFO [recidive] Found 203.0.113.2
  fail2ban.filter [2202]: INFO [plesk-postfix] Found 203.0.113.2
  fail2ban.filter [2202]: INFO [plesk-postfix] Found 203.0.113.2
  fail2ban.actions [2202]: NOTICE [plesk-postfix] Ban 203.0.113.2
  fail2ban.filter [2202]: INFO [recidive] Found 203.0.113.2

Cause

Fail2ban is blocking connections to the server.

Resolution

1. Log into Plesk.

2. Go to Tools & Settings > Mail Server Settings.

3. Check what is specified in the field Installed IMAP/POP3 server: Dovecot or Courier-IMAP.

4. Go to Tools & Settings > IP Address Banning (Fail2Ban) > Jails.

5. Click on plesk-dovecot or plesk-courierimap, depending on the used IMAP/POP3 server found in step 3.

6. Click Change Settings.

7. Adjust the parameters IP address ban period and The maximum number of failed login attempts: increase the number of failed attempts and/or the ban period.

Also, if the customer affected by the issue uses the permanent IP address, consider adding it to Trusted IP Addresses in Tools & Settings > IP Address Banning (Fail2Ban).