The problems with sending and receiving are strictly related to network connectivity, firewall, server availability and login credentials.
1. Network Connectivity
You should check if the Internet is working good on your computer. If you're on PPP0E, disconnect and reconnect.
2. Firewall
Make sure the software is properly configured in the firewall application installed in your computer. If you don't have one, try turning off Windows Firewall from Start > Control Panel > System and Security > Windows Firewall.
3. Server availability
It's possible that the server used to send emails is down for maintenance or it has problems. Contacting the support team for the email service used will get you additional information.
4. Login credentials/configuration
Access the software's settings and double check the connection parameters. It's possible that the software isn't configured properly.
