You need to create a new Address book or use a suitable address book to store the contacts. Then you can create subgroups known as 'Mailing Lists' which can be used to select a number of contacts in the select address book to send one email to several contacts. An address book can have several Mailing Lists and a contact can be in more than one Mailng List. The manual method is select the folder/subfolder select email to see contents. In the header click on the 'From' email address and it gets added to Personal address book, click on Edit Then you can select the desired address book to use or click on 'Edit Details' to access the address book contact information. If you were collecting all email address you were 'sending' emails then you can choose to collect them in any address book, but the default is 'Collected Addresses' address book. Have you ever sent any emails to those people - the particular group of clients? Maybe you already have collected the contacts email addresses, but are not aware of it ? There is an addon extension you can try : It says "Collects email addresses (sender, recipients, reply to, or all together) of all messages in a folder (and subfolders) and saves them in a new address book. Richtclick on a folder and choose 'Collect Addresses'." * https://addons.thunderbird.net/en-US/thunderbird/addon/collectaddresses/