From MozillaZine Knowledge Base
Mail merge is a general term for merging information from a database with a template message, which is sent to many recipients, so that each recipient gets a personalized message.
Mail merge does two things:
If you only want the first of these—to send exactly the same message to many people, so that they do not see each other's e-mail addresses, then you probably do not need mail merge—you can almost always do that using Bcc.
Thunderbird does not support mail merge directly. If you need to send personalized messages, here are some approaches that you might want to consider.
Using an extension
Two extensions implement this feature:
Both add-ons work in a similar way:
Using a word processing program
OpenOffice Writer can read a Thunderbird address book (or some other source of addresses), merge the data with an OpenOffice document to create personalized messages, and send the messages directly to your SMTP server.
There are three steps in OpenOffice:
Using a mailing list program
A mailing list program, (for example, the open source PHPList), can merge information from a conventional database with a template message. In general, a specialized program like this is more appropriate for large lists, large messages, or other requirements like scheduled mailings.