First of all create a backup of your Thunderbird profile. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

Then compact the folder Inbox. Right-click Inbox in the folder pane - Compact

Set up Thunderbird to automatically compact folders. http://kb.mozillazine.org/Compacting_folders

If Inbox is still too big, archive messages. https://support.mozilla.org/en-US/kb/archived-messages

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

Thank you very much, I will try this and i will come back for a feedback.

Hello again, I tried to compact the INBOX folder but i receive an error that i don't have space to complete this task. I must say that i have only 4 Mb left free on drive C. What i did not understand, whem i backup my profile, i must to copy in another location only the .ini file or the entire folder named PROFILE? If i must do that, i must to copy 116 Gb?

... i have only 4 Mb left free on drive C.

Then you'd better free up some space on your C: drive. Running Windows with that little free space left on the system partition is asking for trouble.

... or the entire folder named PROFILE? If i must do that, i must to copy 116 Gb?

That's the whole point of creating a backup.

Thank you very much for your help. I did it!!! Have a great day!

When your problem is fixed can you mark the thread as 'Solved' please? Thank you.