Leon,
Are you using Microsoft Outlook or some other program to send email out? Or are you using a browser-based email client?
It's most likely not allowing you to send out, because you're traveling and using a 3G mobile Internet connection, am I correct? ISP's don't like you using (relaying) through their mail servers if you're connected to some other ISP. Example, if I'm with Optus, and then connect via someone's wireless network that is not with Optus, I won't be able to send email out.
Which 3G ISP are you with? You will need to change your outgoing/SMTP server. For example, I recently got a Telstra BigPond 3G USB modem to connect to the web with when I'm out imaging. To send email out, I had to set my SMTP/outgoing email server to mail.bigpond.com and everything works.
H
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