So I've tried to send an email to about 1000 addresses and I keep receiving failure notice.
Does roundcube limit the number of email address I can send to? And if so, how manu is it?
by default there is no limit, see:
// Maximum number of recipients per message. Default: 0 (no limit)
$config['max_recipients'] = 0;
in your roundcube config. the restriction could be coming from your SMTP server