The technique outlined here comes from Lifehacker: How to use Gmail as your SMTP server. But the instructions are a little outdated as gmail has changed the layout of its settings pages, so I've updated them here.
The issue is that, when you're away from base, you can't always send emails from your email client because you're on the wrong ISP. Some ISPs allow it, some don't. But, if you have a gmail account you can set it up as your SMTP server to use from anywhere. It works in three stages.
First, set your client to use gmail SMTP.
Second, the From field in the emails seen by the recipient will point to gmail, but by changing the settings in gmail you can make it point to your original address.
Third, set gmail to forward the mails it receives to your original.
For the first stage:
1.1 for Thunderbird, go to Tools, Account Settings.
1.2 In the left hand pane scroll down to "Outgoing Server (SMTP)", and click on that.
1.3 In the right hand pane click on Add...
1.4 In the Server Settings window, put a name in the Description - "gmail", for instance. For Server Name put "smtp.gmail.com". Check the "Use name and password" box, and then in the "User Name" box, put your gmail username. It needs to be the whole thing - firstname.lastname@example.org. Finally, in the "Use secure connection" line of radio buttons at the bottom, click in the "TLS" button. Click OK.
1.5 Thunderbird should use whichever SMTP it finds working, but to make sure, click on the gmail address, and then click "Set default". Thunderbird will then use the gmail SMTP until you set the default back to your original. But note that replies to emails you send this way will go to gmail.
But you can make it look better, and work better. You can make it say it comes from your original address, and you can set gmail to forward the mails it receives back to your original address.
So, for stage two:
2.1 open your gmail account in your browser
2.2 go to Settings at the top right
2.3 in Settings, go to Accounts on the top tabs
2.4 click "Add Another Email Address"
2.5 enter the address you want in the From field in the Address box; click "Next Step".
2.6 click "Send Verification", and follow the instructions to verify the account (an email will be sent to your account; click the verification link in that email, or enter the code in the confirmation code window.
2.7 back at the Settings page you will now have your Gmail account and your other account listed. The Gmail account will have the word "default" to its right, and the other account will have a link called "make default". Click the "make default" link. This account name will now appear in the "From" field.
To complete the process, have gmail forward your mail to your other account. This is stage three:
3.1 Back in the Settings page in gmail, go to the "Forwarding and POP/IMAP" tab.
3.2 Click the radio button beside "Forward a copy of incoming mail to", and enter your email address in the box that says "email address".
3.3 scroll down to the bottom of the page and click "Save Changes".
You are now good to go.
You don't even need to change it all back again when you get home, but it's neater if you do.