Sending an email to a list of addresses in a spreadsheet.

In order to send an email to a list of people you have in a spreadsheet, open that spreadsheet (e.g. Excel, Google Sheets) and highlight the email addresses. Do not highlight the heading if applicable.

Press Ctrl+C (PC/Windows) or Cmd+C (Mac)

Open a new email in Gmail. Click "BCC"

Press Ctrl+V (PC/Windows) or Cmd+V (Mac)

Continue composing your email as usual.