there is no one answer to this question.
the minimum _database_ requirement would be an email address.
But you need to take into consideration the mailing audience settings.
If you set the audience to use the users first name and it is empty, that user wont be in the audience.
you also need to take into account the email you configure, and what user tokens you are using.
also, if you leave the password column blank, the user will not be able to login to change any of their settings or to opt out of the mailing, opt in to new mailings that you offer.
to just send a mailing, you only need the email address, but other parts of the whole application need other aspects of the database to have values in order to function.