

You are merely enhancing an existing account. This doesn't require you to add another account or use a different SMTP server. Want to create a set of customized settings in order to switch which SMTP server you use, whether the message is encrypted, what reply-to address to use etc.Want to hide the fact that you're using something like Gmail's Mail Fetcher to centralize messages from multiple accounts.Have multiple accounts and don't want to have to worry about what folder is currently selected in the folder pane.Forward messages from another e-mail address (either an account or a forwarding service such as Bigfoot).Use plus addressing to create disposable email addresses to help deal with spam.Have mail sent to multiple email addresses in your inbox.

This lets you select the desired From: address from a drop down list when composing a message, have the From: address default to whatever email address a message was sent to when replying, or use a different set of customized settings when sending a message. You can tell Thunderbird to use more than one identity (personality) per mail account. This article was written for Thunderbird but also generally applies to current versions of SeaMonkey.
