There is a limit of "approximately 60" discrete addresses in Thunderbird. Your email provider may also set a limit. http://kb.mozillazine.org/Limits_-_Thunderbird#E-mail_addresses You can use a Mailing List to get around Thunderbird's built-in limit. But this may not help if your provider sets a limit. Thunderbird's natural mode is to have one address per line. Enter an address, press return to move to the next line. This procedure allows Thunderbird to validate entered addresses against the address book. You ''can'' enter multiple addresses into one box. Use commas to separate them, but understand that this will prevent it doing any address validation, and addresses added this way will be shown in red to warn you that validation has failed. If you do press return, it will unfold your stacked addresses into the preferred one-per-line format. You can grab and drag the horizontal divider between the addressing box and the message text box to move it up and down to change the number of addressees displayed.