What do Sent, Not sent and Failed mean in my store's email log?

Go to Settings → Web Settings → Email Delivery Logs. The Email log lists every email your store tried to send, with When, To, Subject, Type, Recipients and Result. The Result can be:

• Sent — the email went out.
• Not sent — limit reached — your monthly email allowance was used up. A red note at the top shows how many were stopped, with a link to buy more emails or upgrade.
• Not sent — no reCAPTCHA — a contact form email was blocked because the form has no verified reCAPTCHA. Add the reCAPTCHA field to the form and set up your keys.
• Failed — sending was tried but did not succeed.

Use the drop-down at the top to show only one result, and Previous / Next to page through older emails.