The Send Now button/option will be greyed out if something is wrong with what is in the To: section, like if there is nothing in the To, or what is there is not a valid e-mail address or a valid mailing list. Something that I've seen before here is that the address book may contain an invalid contact entry (it is missing an e-mail address), that has the '''same name''' as your mailing list. When you type the name into the To: field, Thunderbird automatically chooses the contact with that name, instead of the mailing list with the same name. So check your address books to see if you have a contact with the same name as the mailing list.