if the accounts are Gmail or Outlook, those save to the Sent folder by default. That is, from my experience, even if you set thunderbird to save the sent message elsewhere, a copy still goes to the Sent folder.

david is quite correct. Many servers now auto save sent mail in the server 'Sent Mail' folder.

As a heads up: If you right click on an email and select ' Open' > 'Open message in Conversation' It will open in a new tab and contain all messages within that thread including any responses you made even if they have stored in the 'Sent' folder. So you can see the entire conversation in context.

It means you do not have to save sent emails in the Inbox in order to see the conversation in context. It also means the Inbox mbox file does not become so large. After all, the mbox file is just a single text file with emails appended to it as an when they get downloaded. You significantly increase the risk of potential major loss etc if you keep a lot of emails in just a single text file.

The 'Open message in conversation' applies to any email no matter what folder it's stored in.

This is a rather a good way of viewing a particular conversation as it removes all the clutter of other threads and you can leave it open in case you need to go back and refer to it.

Thanks David and Toad-Hall