If the upgrade passed without errors I can suppose that you have some problem with connecting to SMTP service running by local MailEnable. There might some reason for that:
1. Some connection settings in Horde to connect to SMTP server to send an email are changed from default ones. Solution: revert changes back to defaults.
2. MailEnable SMTP service is stopped. Solution: start MailEnable SMTP service.
3. Some other service is running on port 25. Find this service using command "netstat -aon | findstr 25". It shows the ID of the process and find this process. If this process is NOT MailEnable SMTP then stop this process and make step 2.
4. Some antivirus blocks connection to local port 25.