Just changed from Evolution to Thunderbird 3.1.10. and wanted to share the steps taken.
My platform: ubuntu 11.04 64-bit, desktop.

Step 1.
  • Backup Evolution: Alt-f, Backup evolution settings...
  • Install Thunderbird from Ubuntu Sotware Center and setup Thunderbird with your mail account information. Close Thunderbird, after checking whether sending and receiving works.

Step 2.
Open evolution and move all sub-folders (dragging them) to the highest level. Before going to the next step make sure no sub-folders exist any more. Then close evolution.

Step 3.
  • Open Nautilus.
  • Assure you are in your /home/yourname folder.
  • Press Control-h, now you see all hidden folders and files below your own folders, by example: .aptitude
  • Move to /home/yourname/.local/share/evolution/mail/local/Inbox.sbd
  • Press F3. This opens another panel in Nautilus.
  • In this panel goto /home/yourname/thunderbird/xxxxxxx.default/Mail/ServerName

This servername you can find in Thunderbird account settings (Alt-e, a), go to Server Settings. Here you see the ServerName.

Step 4.
Copy only the "mailbox file" files from your Evolution "Inbox.sbd" folder to the Thunderbird "ServerName" folder. Do not copy the other files like xxx.cmeta, xxx.ibec.index, xxx.ibex.index.data etc.

That's all.

Open Thunderbird and you should find all your evolution mail in your folders. By dragging the folders you can recreate your evolution subfolder structure.

Hope you like it