An email alias is an email address that forwards to another email address. For example, if your primary email address is "email@example.com", you might create a second email address called "firstname.lastname@example.org", which is easier for people to remember. Messages sent to your "email@example.com" account would then be forwarded to your "firstname.lastname@example.org" account.
The ability to create email aliases depends on your email provider. Once you have set it up (probably through your provider's website), you can configure Thunderbird to download and sort messages to your alias account in the same manner it deals with messages to your primary account.
The following example shows how to configure a Hotmail / Windows Live Mail account to place messages to your email alias in your inbox. Other email providers will usually have an equivalent interface.
When you reply to a message that was sent to your email alias, you may want the reply address to be your alias address rather than your primary address. You can use Thunderbird's Identity functionality to configure this.
Instead of storing messages sent to your email alias in the standard Inbox, you can configure Thunderbird to store them in their own folder.
Messages will now be automatically stored in the designated folder on arrival.