All email providers (like Gmail or Outlook) have an hourly sending limit, which usually floats around 80. When sending out a smart mail with over 80 contacts, you will see this pop-up message below:

When you see this message, you can click on the "SEND ALL" option to send out the first batch (around 80 emails).

After the first batch is sent, please wait for about 1 hour and then return to Smart Mail to send out the remaining emails.

After clicking into the smart mail, you will see the pipeline, click on the "Edit Smart Mail" button

Once you are back in the smart mail, click on "Send All" again. It will only send to unsent contacts and will not cause duplicate sends.