You can track the last time of sending an email by creating a custom date field in the module and performing a field update action using a workflow.
Step 1:
1. Go to Setup > Customization > Modules and Fields.
2. Choose the module and edit the layout.
Add the date or date/time field to the module and save the changes.
Step 2:
1. Go to Setup > Automation > Workflow Rules > Create New Workflow Rule.
2. Choose Emails as the module.
3. Choose the initial condition as when an outgoing email is sent.
4. Add condition 1 as apply to all.
5. Choose Field Update as an Instant Action.
6. Choose New Update, select the date field, and set the value as Execution Day.
7. Enter a name for the field update and save the changes.
Similarly, if you want to track both incoming and outgoing email activity time, you can create the same type of workflow rule for incoming emails and choose the same field update as described above.