https://support.mozilla.org/en-US/kb/addressing-email You can not however exceed the number of recipients limit your mail provider enforces. Most ISP's have limits significantly less than 60. One way to get around the recipient limit is to send a mail merge email to each individually. https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/?src=search This might have the effect or dropping you into a "number of emails per hour" limit imposed by your ISP or mail provider. But Thunderbird itself imposed no limits at all.