Normally, when sending bulk emails for a client, you have a carefully maintained master list. You keep track of new subscribers and people who have unsubscribed so you don.t accidentally harrass them.
But sometimes you have to do a one-off send using an Excel spreadsheet that client sends you.
And the client says that the spreadsheet only has active subscribers when really, it has both active and inactive subscribers and they.ve used the Excel .hide. feature to hide the rows of inactive subscribers.
This ensures that, when you export the spreadsheet to a CSV file to send the email, you email everyone and get to spend the entire morning doing damage control and quickly writing an apology email to the inactive people (which further enrages a small percentage of them).
Never trust your inputs.