What
The Last Contacted date field on a Contact record is used to indicate the last time the Contact was interacted with.
Where
Home > Contacts > Contact record > General tab
How
Processes which WILL update the field
The following processes will cause the Last Contacted field to be automatically updated:
A completed Phone Call
A single Email generated from Mercury.
Processes which WILL NOT update the field
The following processes will not update the Last Contacted field:
Emails received
Emails sent from Outlook and then tracked back to Mercury
Sales emails - see this Knowledge article for more information on sales emails: "Communication fields used when reporting".