How do I send to everyone in my address book?

The easy way to add multiple Contact addresses to a message in Thunderbird is to start the message, then display the Contacts Sidebar. Press F9, or use the menu: View|Contacts Sidebar

The Sidebar appears at the left, showing one of your address books. It has "Add to..." buttons at the bottom, so you can select one or more Contacts, and click on the appropriate button at the bottom. You'll see that used this way, Thunderbird creates a new line in the addressing box for each address. The apparent limitation of four addressing lines fazes many new users, and to be honest, when you've added five or six addresses, it can be tiresome to scroll the list up and down to check who's there. You can drag the top edge of the message text pane down to make more room to display addressess. I quite like the Address Close Button add-on, which besides adding a delete option alongside each addressee, gives you a tooltip pop-up summarizing the addresses entered.

You can also right-click Contacts in the Sidebar and select how to add them to the message, and you can drag-and-drop Contacts into the addressing box. This last method has the quirky side-effect of adding multiple selected and dragged addresses all into the same address box, separated by commas, which many new users of Thunderbird seem to expect as normal behavior.

Now, a digression: imagine that a friend has sent you a message announcing something momentous, such as the change of an email address. You notice that your address is just one of about 100 addresses in the "To:" box. It dawns upon you that this same message was sent to all 100 addresses, and now all of them know your email address too. How many of them are as naive or careless as your friend, and send messages out to all and sundry with no thought to their acquaintances' privacy? How long before your address is picked up and added to spammers' mailing lists?

When we wish to send to many addresses but without showing recipients' addresses to one another, there is an addressing option intended to help out: Bcc: ("Blind Carbon Copy") - but please see the footnote.

In the following case:

To: "John Smith" <>

Cc: "Jane Smith" <>

Bcc: "Alan Smith" <>

Bcc: "Betty Smith" <>

...the message will be sent with John Smith and Jane Smith visible. John and Jane won't see that the message was also copied to Alan and Betty. In fact, neither Alan nor Betty will see their own names in the message header, so neither will be aware that the message was sent to the other.

If we'd sent it like this:

Bcc: "John Smith" <>

Bcc: "Jane Smith" <>

Bcc: "Alan Smith" <>

Bcc: "Betty Smith" <>

...the message would arrive at all four recipients addressed to "undisclosed recipients".

The fact that a message has arrived in, say, Alan's inbox but not ostensibly addressed to him may cause some trouble; he may aggressively filter out any messages not addressed to him. So Bcc: always carries some small risk that some of the intended recipients will ignore it.

Right, now we know how to avoid showing all the addresses to each recipient, how do we add all of our address book Contacts to the message?

Using the Contacts Sidebar as shown above, you could simply click on a Contact, then use ctrl+a (hold down control and press a) to select all of the displayed addresses, then add them to the addressing box. You could then select another address book in the Sidebar, and repeat, until all of your correspondents have been added.

The bad news is that Thunderbird has an internal limit and at 60 or so discrete addresses, will refuse to send your message. This is a simple and crude way to deter spammers from using Thunderbird. So you could batch up your sending into groups of 50 or so each time. Even if you avoid Thunderbird's limit, you may find that other mail servers handling your messages apply their own limits too.

One way to work around Thunderbird's limit is to use a Mailing List, similar to a Group or a Distribution List in other email clients. You add the Mailing List to your addressing box, set it to Bcc:, and each Contact stored in the Mailing List should receive their own Bcc:'d copy. The same comments about single addressees apply; names in the To: or Cc: box will appear to all recipients, and Bcc:'d recipients (probably) won't see their own names.

Mailing Lists are fine for regular mailings to a consistent set of recipients, but it doesn't seem sensible to set up a Mailing List for a one-off notification task.

I think the right way to do this kind of thing is to use Mail Merge - this will create a separate message for each addressee, each message personally addressed to its recipient. You avoid the impersonal "undisclosed recipients" of the Bcc: method, and reduce the risk of users filtering out messages which are not expressly addressed to them.

Bcc: doesn't always work as we'd like. The specifications suggest that it should work as I've outlined above, but the same specifications allow for Bcc:'d recipients to be shown each others' names. Before using Bcc: in anger, send a message to yourself, using Bcc: and as many different addresses as you can muster, and check what the received messages say about who they were sent to. If all the addresses were in the Bcc: box and the received messages say To:"undisclosed recipients;" then you can be reasonably sure that your correspondents' privacy will be preserved. If your smtp server doesn't  work as you'd like, you need to find another one that does, or switch to using Mail Merge

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