Invalid email address. Check if all the addresses listed as mail recipients are correct. If the message contains a wrong or invalid email address, the message will not be sent.

Incorrect SMTP settings. Verify if the settings used for outgoing messages is correct. Using the incorrect server address or authentication method may prevent emails from being sent

Offline status. Evolution may be offline. Check on the Send and Receive button; if it is greyed out, then you are offline. To go online, go to FileWork online. You should now be able to use the Send and Receive button.