As TheChair says, most ISP-based email services configure their SMTP servers so that they'll only accept incoming mail from IP addresses linked to their on internet service. So, for example, if you've got an email address such as ask100@firstisp.co.uk and you change your ISP to a different one your old email adress will no longer work. Similarly, if you connct to the internet via a public free wifi service, your ISP's SMTP server will refuse the connection.
That's why it's always unwise to use an ISP-based email address. It makes far more sense to use an independent free email service, ensuring that (if you want to use an email client, such as Outlook, rather than web-based access) you choose one which supports POP3 or IMAP connections. (Both Gmail and GMX can do that).
It's possible (although, I suspect, unlikely) that your ISP-based service might accept an 'authenticated' connection. It's easy enough to check anyway; simply go into your account seting in Outlook and tick the 'authentication' option.
Otherwise (unless you switch to a better email service, such as GMX), you'll have to use web-based access to send mail when you're away from home.