Multiple identities per e-mail account
- This article was written for Thunderbird but also generally applies to current versions of SeaMonkey.
You can tell Thunderbird to use more than one identity (personality) per mail account. 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. This is useful when you:
- Have mail sent to multiple email addresses in your inbox.
- Have a catch-all account for a domain
- Have aliases.
- Use plus addressing to create disposable email addresses to help deal with spam
- Forward messages from another e-mail address (either an account or a forwarding service such as Bigfoot)
- Have multiple accounts and don't want to have to worry about what folder is currently selected in the folder pane
- Want to hide the fact that you're using something like Gmail's Mail Fetcher to centralize messages from multiple accounts
- 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. when sending a message.
This doesn't require you to add another account or use a different SMTP server. You are merely enhancing an existing account.
How to add another identity:
- Go to your account settings.
- Choose the account to which you wish to add additional identities.
- Click the "Manage Identities..." button (on the lower right).
- Click the "Add..." button and then add the necessary information for the additional identity for the account.
- Repeat as necessary for additional identities.
When you write a new message, Thunderbird uses the default identity for the account you are working in. When you reply to a message, Thunderbird uses the identity that matches the address the message was originally sent to, if it can find a match in the account you are working in. Otherwise it uses the default identity. Set mailnews.reply_to_self_check_all_ident to true using the Config editor if you want it to check the identities for all accounts when replying.
You can select any multiple identity when replying, forwarding or composing a new message by selecting it in the From drop down list box. That identity (except for the From: address) defaults to the accounts settings but it can be modified to use different settings such as a different signature, draft folder, "sent" folder, SMTP server etc. using the Edit button in the Manage Identities window.
Thunderbird will let you select any accounts From: address in the From: drop down list. However, in most versions that identity is not actually used unless its either the current accounts identity, a multiple identity defined for the current account or you set mailnews.reply_to_self_check_all_ident to true.
Messages sent using mailing lists frequently don't specify what address the message was sent to in the To: or Cc: headers. It might be buried in one of the Received: headers, or in mailing list specific headers such Delivered-To: or Envelope-To: that Thunderbird has no knowledge of. In that case, Thunderbird doesn't know how to match the address the message was originally sent to and will typically select the default identity. If that happens you need to manually select the appropriate identity from the list box. If you use a separate folder for mail sent to each mailing list one workaround would be to use the Folder Account extension to specify what identity to use when a specific folder is selected.
The Gmail SMTP server will ignore whatever From: address you supply unless you add it in the Gmail web page at Setting -> Accounts -> "Add another email address". The AOL and AIM SMTP servers won't accept a email address that doesn't match the the screen name used to authenticate when sending.
Thunderbird 2 changed the behavior when replying to a sender, where the sender is one of your multiple identities. It used to reply to that identity, now it uses the default identity. According to this bug report its by design. Set mailnews.reply_to_self_check_all_ident to true to make it check all of your identities.
Folder specific settings
The Folder Account extension lets you select what account or identity will be used when composing, replying, or forwarding mail based on what folder is currently selected. It also lets you specify the default To: address when composing a message based on the currently selected folder. Right click on the folder and select properties in order to configure the account/identity and/or To: address.
You can still select a different identity from the From: drop down list box if you use this extension.
Recipient specific identities
The Flexible Identity add-on lets you configure what identity to use based on who you are sending a message. It supports both simple filters and regular expressions in its rules, and has a option to warn you whenever it applies a rule. This can be useful if you are worried about accidentally using a personal email address when replying to work related email (and vice versa).
If occasionally you want somebody to reply to a different email address you don't have to specify a different From: address. Just choose Reply-To: using the To: list box near the top left corner of the compose message window. Don't forget to also add a To: or Bcc: address.
S/MIME and Enigmail
The S/MIME certificate only applies to the accounts default identity. You need to copy the settings from the default identity to mail.identity.idN.encryption_cert_name , mail.identity.idN.encryptionpolicy, mail.identity.idN.sign_mail , and mail.identity.idN.signing_cert_name . 
- Installing an SMIME certificate
- Multiple SMTP servers - Thunderbird
- Mail concepts
- Modify Thunderbird settings
- Secondary accounts
- The Correct identity extension may make it easier to choose an identity. It supports aliases.
- The Flexible Identity extension checks the list of recipients before sending a message and displays a warning if the outbound email address or domain does not match your needs.
- The Folder Account extension lets you associate user accounts and identities with specific folders.
- The Identity Chooser add-on forces you to choose the From: address first, before composing a message. It also uses colors to help you chose the right email address.
- The Identity Reminder extension might be useful if you're worried that you will accidentally reply using the wrong email address. Another solution would be to use the BorderColors extension to color code the compose message window based on the current identity.
- The Identity Select extension lets you select your identity from the Write button.
- The EditSender extension lets you change both the name and email address of the sender to whatever you want (on the fly) when composing a message. Right click on the From: line and select "edit sender details for this message" to edit it.
- The NoComposeAccount extension can be configured to remove the default From: address, forcing you to explicitly choose one, or default to the default accounts From: address regardless of what account/folder is selected in the Folder pane.
- The Virtual identity extension is another way to modify the From: address on the fly while composing a message. Left click on the From: line to directly edit it.
- The Freeform From and FCC on Compose extension is the most well known of the extensions to modify your email address on the fly when composing a message. Unfortunately the author is no longer reachable and their web site is gone. The Add-ons Mirror has a copy, and periodically bumps the version limit so that it still installs.