You have to enter at least one address in the TO field, so unless you want to disclose one address to everyone the mail is sent to it's best practice to put your own address in the TO field. the problem with this is it will look a little strange when people receive an email that looks like it's not addressed to them. to get round this if you create the contact on your address book named undisclosed recipients with your email address then when people receive the email they can straight away see it's been sent to undisclosed recipients.