Open Logs in the ADMIN group. Use Email logs to see every email's status, and Audit logs to see who signed in and what changed.

Email logs
The boxes at the top count Pending, Failed, Blocked and Sent. Below them, each row shows the time, status, lead, user, recipient, subject, priority and attempts. Statuses include PENDING, PROCESSING, SENT, FAILED, BLOCKED and CANCELLED.
Search by subject or recipient, and filter with All Statuses, All Types and Advanced filters.
Audit logs
Click the Audit logs tab. The boxes at the top show Failed logins and Lead changes. Each row shows the timestamp, the action, such as User Login, the user, the entity, the IP address and a status.
Search by user or entity name, and filter with All Actions, All Entities and Advanced filters.

Buttons at the top
Live keeps the page current, Refresh reloads it, and Export saves the list.
Other logs on the same page
SMS logs, Call logs and Power dialer are tabs on the same page. See How to check if a text message was delivered in FlowForce and How to review your call history, missed calls and voicemails. For queue totals, see How to monitor your email queue in FlowForce.
If it did not work
I do not see Logs. It is in the ADMIN group. Your role may not include it. Ask an admin.
An email is Blocked. Check the lead's opt-out settings, and your sending limit. See How to check your email sending limit and Inbox ramp in FlowForce.
I want to know who changed a lead. Open Audit logs and search the lead's name.