Greetings,
ListMail could have caused this if you have that number of users and set up one or more followups on the list(s) containing those users.
When a followup is added all users on the list are automatically set to start the sequence the next day, providing you have not chosen the "set all users to new delay" option, which would delay delivery by a number of days.
If you added more followups (Seq #2,3, etc) these will also be sent in the following days. As of v1.86 You can use the option "Reset Users" to set all users to the chosen followup. Set your users to ONE more than your last Seq # and your users (on that list) will not receive followups next dailymail.
I've made a note to be more descriptive and up front about how and when followups will be delivered (ie. a note in the Dailymail setup page) to try to avoid this in the future. I'm sorry it had to happen to you. I recommend that you make contact with your host and inform them about your email marketing practices (ie. 100% opt-in from your web site) to alleviate concerns. If your host is unwilling to let you host ListMail for your number of users feel free to email me at dean@listmailpro.com with some details about your operation and I will consider you for my optimized-for-email hosting. Cost is typically $1/mo per thousand list members and you can email your lists once or twice a day.
Regards