Assuming they means “percentage of emails sent by John Smith to you” and assuming you have a reasonable email client, it’s should work like this:
Run a search of all emails from John Smith during the last month. Count ’em.
Add another term for the search: the word “sustainability”. Scan what you got, discard what you don’t want, count what’s left.
Divide the second number by the first number.
Assuming they means “percentage of emails sent by John Smith to you” and assuming you have a reasonable email client, it’s should work like this:
Run a search of all emails from John Smith during the last month. Count ’em.
Add another term for the search: the word “sustainability”. Scan what you got, discard what you don’t want, count what’s left.
Divide the second number by the first number.