If you sent to more than 60 contacts, some of them may not have received the mail, unless you used a mailing list: http://kb.mozillazine.org/Limits_(Thunderbird)#E-mail_addresses A mailing list would be as easy to create as adding contacts one at a time, and could be reused for future mailings. Or, open the Contacts Sidebar (F9 in the Write window), select multiple contacts while holding Ctrl, then click an Add to button at the bottom of the sidebar.