If you're experiencing issues connecting to a mailbox via Roundcube webmail or any email client using IMAP after installing Plesk Premium Email, there are several steps you can take to troubleshoot and resolve the problem:

1. Check Server Status:

   • Ensure that all necessary services (e.g., IMAP, SMTP) are running on the server. You can do this through the Plesk panel.

2. Verify Mailbox Configuration:

   • Double-check the mailbox settings, including the username, password, incoming and outgoing mail servers, and port numbers. Any incorrect information can prevent successful connections.

3. Check Firewall Rules:

   • Verify that the server's firewall is not blocking the IMAP port (usually 143 or 993 for secure IMAP).

4. Verify IMAP Service:

   • Make sure that the IMAP service is running on your mail server. You can check this in your server's service manager or through the command line.

5. Check for Disk Space:

   • Ensure that there is sufficient disk space available on the server, as a lack of space can cause various issues, including connection problems.

6. Test Connectivity Locally:

   • Connect to the IMAP server from the server itself using a command-line tool like telnet to ensure that the IMAP service is responsive.

7. Check Error Logs:

   • Review the mail server logs for any error messages or warnings related to IMAP connections. These logs can provide valuable information about what might be causing the issue.

8. Verify SSL/TLS Settings:

   • If you're using secure connections (which is recommended), ensure that SSL/TLS is properly configured on both the mail server and the email client.

9. DNS Configuration:

   • Verify that the DNS records for your mail server are correct and properly propagated. Incorrect MX or DNS records can lead to connection issues.

10. Check for Account Lockouts:

    • Make sure that the account you're trying to access is not locked due to too many unsuccessful login attempts.

11. Test with a Different Email Client:

    • Try connecting to the mailbox using a different email client to see if the issue is specific to a particular client.

12. Contact Support:

    • If none of the above steps resolve the issue, it may be necessary to contact your hosting provider or Plesk support for further assistance.

Remember to document any changes you make during the troubleshooting process, and always back up critical data before making any modifications.