1. Unknown Host Error

1. Description: This error occurs when the mail client cannot resolve the hostname of the mail server to an IP address. The issue typically arises from DNS resolution problems, incorrect mail server configurations, or network-related issues.
2. Possible Causes: 
   
1. Incorrect Server Address: The hostname provided for the mail server might be incorrect.
   
2. DNS Resolution Failure: The DNS server cannot resolve the provided hostname to its corresponding IP address.
3. Firewall or Security Restrictions: Firewalls or security software block DNS queries or access to the mail server.
3. Troubleshooting:
   
1. Ensure mail server hostname provided is correct.
   
2. Run ping command and ensure mail server host is accessible from Applications Manager installed server.
   
1. ping <mailserver_host>
   
3. Verify that no VPN, proxy, or network restriction is causing connectivity issues.
4. Ensure firewalls or antivirus software is not blocking the server access.

2. Connection Refused/Unable to Connect/Connection Timeout

1. Description: The "Connection Refused/Unable to Connect/Connection Timeout" error occurs when a mail client (such as an email app or server) cannot establish a connection to the mail server. This error usually indicates that the mail server is either unreachable or has rejected the connection attempt outright.
2. Possible Causes: 
1. Server is Down or Unreachable: The mail server might be offline, undergoing maintenance, or experiencing network issues.
2. Incorrect Server Address or Port: The client may be configured with the wrong hostname, IP address, or port number, preventing a successful connection.
3. Firewall or Security Software Blocking the Connection: Firewalls, security software, or network policies may block access to the mail server’s ports.
4. Protocol Mismatch: The client might be trying to connect using the wrong protocol (e.g., using POP instead of IMAP).
5. Server Load or Connection Limits: If the server is handling a high number of requests, it might temporarily refuse new connections until resources become available.
3. Troubleshooting:
   
1. Check Server Status: Ensure the mail server is online and reachable.
   
2. Verify Server Address and Port: Confirm correct hostname and port for SMTP, IMAP, or POP.
   
3. Review Firewall/Security Software: Check that firewall or security settings allow the connection.
   
4. Confirm Protocol: Ensure correct protocol is selected for IMAP/POP configurations.
   
5. Proxy configuration: Check if any proxy configuration is required for connecting to mail server.
   
6. Timeout: Try increasing the timeout and try again.
   

3. Unsupported or unrecognized SSL message

1. Description: This error indicates a failure in establishing a secure (SSL/TLS) connection between the mail client and the server. It typically occurs when the client attempts to use SSL/TLS on a server or port that does not support it, or when there is a mismatch in protocol configurations.
2. Possible Causes: 
   
1. Incorrect Port Configuration: The client is configured to use SSL/TLS on a port that does not support it.
2. Non-SSL/TLS Enabled Server: The mail server does not support SSL/TLS or has it disabled for the selected port.
3. SSL/TLS Certificates issue: The server’s SSL/TLS certificate is invalid, expired, or misconfigured.
3. Troubleshooting:
   
1. Ensure the correct port and protocol were selected.
   
2. Below are the default SSL/TLS ports for Mail server
   
1. SMTP with SSL/TLS: Port 465
   
2. IMAP with SSL: Port 993
   
3. POP3 with SSL: Port 995
   
3. If TLS is enabled, Select TLS Enabled option.
   
4. Try adding the Mail server monitor with SSL/TLS option disabled and providing non-SSL port.
   

4. Unexpected error