If you are able to send email successfully via webmail, but not from a desktop application (e.g. Outlook) then it suggests that the issue lies with your ISP. An increasing number of ISPs are blocking SMTP connections on port 25 - this is the default used around the internet to send email, and an ISP blocking your connection to us on port 25 tends to be the main cause of any issues you might have sending email.
If you are on Windows, or our clustered hosting platform you can get around this by looking in the 'Advanced' settings tab of your account settings in your email client. You'll need to change the port number to 465 on your Outgoing (SMTP), as well as checking the SSL tickbox. You will only need to change the outgoing/SMTP settings - you can leave your incoming settings as they are.
One thing to bear in mind, is that connecting via SSL may generate a certificate mismatch error if you have your outgoing mailserver set to mail.yourdomain. If you get this, you should change your outgoing mail server to 'mail.gridhost.co.uk'.