For outgoing e-mails you can configure to cc or bcc a copy to a specified address. Configure it under Tools | Account Settings | [account] | Copes & Folders | Cc/Bcc these e-mailaddresses.

For incoming e-mails you can create a filter that matches all messages and forwards the message. Tools | Message Filters…

But the problem is I don't want my employee to know that all their outgoing mails are being forwarded to some other account. With the above solution, employee will be able to know about bcc part and they can even make changes.

Are you allowed to do that? I think it would be unlawful in some jurisdictions.

To do this you need to intercept outgoing messages at the outgoing SMTP server. Businesses need business grade solutions and you're probably trying to get by with a configuration designed for home users.

Your email provider may have the abilty to set up filters at the incoming and outgoing servers.

When you do it in Thunderbird your users can always see what's going on, but it's possible to prevent them from making changes to the configuration.

See this knowledge base article if you want to do this: http://kb.mozillazine.org/Lock_Prefs

Thank you guys for sharing your views and suggestions. I wanted to know, since it is open source, can we develop our own extension with the help of available APIs of Thunderbird, having all the required features ? Assumption: Jurisdictions allow us to do.

I think you would be far better served to explore the capabilities of your mail server. Making copies of all mail traversing the server should be a small issue. Unlike trying to make a mail client send everything twice etc.

Currently I use rediffmail server and rediff doesn't provide such feature which I want. There are other mail server like IBM lotus note, which has feature to make a copy of all outgoing mail and sending it to another account but that is too costly for us. So I am looking for anything which would be around my budget and also fulfill the requirements.