Are yous using Outlok the client or Outlook Web access ?

If you are using the Outlook client, i suspect that the problem is that your ISP SMTP server requires that you are connected to them as an ISP to send email, quite a common issue.

Most ISP's nowadays require that if using SMTP or POP3 that you are connected to them as an ISP which whilst travelling isnt always the case.

You can configure Outlook to access web based email (http) so if your ISP allows you to collect your email via the web you can.

For example i use Pipex as my ISP, i cannot use my outlook program (client whilst travelling to use that email account which i can from home.

I can however go to netmail.pipex.net to access is online via http or alternatively you can set up a http email account in outlook depending on the version you are running.

hope this helps