Prompt: Some of your emails cannot be sent.
Possible reason, messages to a certain email address is blocked, either incorrect email address and cannot be sent.
To further check, please try to tick the field "Enabled SSL" and save settings, it should work.
If not, you need to check the settings with your tech support, ie the SMTP Server and SMTP Port.
Workaround: Alternatively you can switch to default LodgeiT SMTP server, via Gear Wheel > Settings > Email.