If you are able to receive emails but can't send, that means that your outgoing server isn't correct for the ISP you're currently using. Every internet service provider has their own outgoing server, and they will not allow you to use the server unless you're currently using their connection.
To start sending emails again, you'll need to change your outgiong mail server to your current ISP. Every ISP's outgoing mail address is different, but they're generally something like mail.yourispname.com.
For example, if you were with Optus for your internet connection, your outgoing server might be mail.optusnet.com.au. If mail.yourisp.com does not work and you are still unable to send emails, you will need to call your internet service provider to find out what their outgoing mail server is.
