Following are the tasks to perform if you receive "Connection to storage server failed" error message.
1. Check with the roundcube log located at /var/log/plesk-roundcube/errors
2. Check whether the Imap service is running in the server. You can check whether port 143 is listening
3. Check whether selinux is disabled.
4. Check with the disabled functions
5. Check with user permission issue for /var/qmail/mailnames/* [ usally popuser:popuser ]
