There is an addon, mailmerge, that lets you send an email and have individual copies go to recipients. But to do this with a higher degree of success, I encourage considering a mailing list company, such as Mailchimp (or others). Otherwise, you run the possibility of having your emails rejected or considered spam, or being blacklisted. There are many websites with info on mailing lists and their management. Thunderbird can send the messages, but the problem would more likely come from your email provider or recipients' email provider.