Hi,
I succeeded to add a date to the subject by modifying the sendmail.py file, an example:
yesterday = datetime.datetime.now() - datetime.timedelta(days = 1)
message['Subject'] = subject + " " + yesterday.strftime("%d.%m.%Y")
It is located in the /opt/splunk/etc/app/search/bin/ directory.
It is a python script and it can be modified with any text editor. Keep in mind that it will be probably overwritten on the next Splunk update.
Where is the sendmail.py file, and how do I modify it?