If you use Outlook panel (and not enable EAC), you can "Log Email" manually to Salesforce for email received and sent out. Both emails received and sent will be stored both as EmailMessage (prefix 02s) record and Task record (prefix 00T).
How to differentiate email send from Salesforce and manually logged from an email client?
You will not find any difference on the Task object, both Type and TaskSubType will populate with "Email". But there are some differences in Email Message object. Check out this query:
SELECT Id, ActivityId, FromAddress, ToAddress, FromName, IsClientManaged, MessageIdentifier, Subject, TextBody FROM EmailMessage
Row 1,4,5 - email manual log from Outlook
Row 2,3 - email sent from Salesforce
As you can see, IsClientManaged and MessageIdentifier are different.
Note: using My Email to Salesforce service (BCC) will not create Email Message records, but only Task.
Email in Lightning activity component
All email logged to Contact or Lead will be shown in the Lightning under activity component.
For manually logged email to Salesforce, click the email will open EmailMessage record, you will be able to Reply All, Reply, or Forward the email from that panel. Hover mouse over the email will show the URL where it will be landed, example: https://mydomain.lightning.force.com/lightning/r/02s2H00001BMfIgQAL/view
For email from BCC email services, click the email will open Task record, there is no option to reply, reply all or forward as the record is a task. Hover mouse over the email will show the URL where it will be landed, example: https://mydomain.lightning.force.com/lightning/r/00T2H00007KIQvsUAH/view
For EAC email, click the email will open the email in that panel, you also able to Reply All, Reply, or Forward the email from that panel. The system also shows if the email is shared with everyone or a group or not shared. Hover mouse over the email will not show any URL, click "View full email" will open the full email as email stream.