There are a number of issues with Thunderbird 3 which may cause errors such as...
"The message could not be sent because the connection to SMTP server ***.***.co.uk timed out"
There are a few things you can try, before reverting to an earlier version of Thunderbird or using a different client entirely.
1. Open Thunderbird 3.
2. Choose Tools > Accounts… from the menu bar.
3. Click on "Outgoing Server (SMTP)". You should see something like this...
If your User Name is not your full email address, or the Port, Secure Authentication or Connection Security Settings vary from those shown above, this may be the problem.
4. To change the User Name, Secure Authentication or Connection Security settings, click on 'Edit' and change them in there. Save the changes then restart Thunderbird before testing.
If your settings for User Name, Connection Security and Secure Authentication already match our correct example above and you still are getting the error, you can try changing the Connection Security to "STARTTLS" in SMTP Settings (as above). Some users have reported this being the only way they could get TB3 to work correctly. Remember to save the new settings and restart Thunderbird 3 before testing again.
1. Increasing the timeout settings
Go Tools > Options then select "Config Editor"
After the "There be dragons" message has been OK'd, you can now type the word "timeout" into the filter box. Now double-click on the entry "mailnews.tcptimeout".
Change the value to 300 and ok.
Restart Thunderbird and retest.
2. Test for Anti-virus or Firewall interference
If this is the first time you have used Thunderbird 3, it could be that your anti-virus software or firewall is interfering with its correct operation. Unfortunately, instructions for configuring different firewalls and Anti-virus products vary. A simple but risky way to quickly test for this kind of issue is to very briefly disable the anti-virus and/or firewall software while testing Thunderbird again. We would not recommend you do this, for security reasons, and you may wish to consult an IT specialist.