When traveling, you are away from your normal ISP connection and dependent upon the hotel or a hotspot for email access. We usually find that reading email is no problem, but sending can be a nightmare -- why?
Many users are now using GMAIL so this will not apply, but for those with email related to their ISP, it's a sure bet that your SMTP setup will fail. This is due to the necessity to disallow what is known as an Open SMTP Open Relay. If this were allowed, we would even far more spam in our inbox's.
If you can find your ISP Webmail link, then you can send from there.