Filtering to display leads who have not been sent an email in X days

Filtering to display leads who have not been sent an email in X days

How can we filter to display leads that have not been sent an email in X days?

Problem:
I have experienced inconsistency with the Email Status 'not sent' filter — sometimes leads who HAVE been emailed within the timeframe are appearing within the filtered results...so I contact Zoho support...

I've been informed that if a lead responds (inc autoresponder etc) then that lead is no longer considered as 'sent'. (strange - why would a reply cancel this out?!)

Example.
  1. I send mass email this morning and get an autoresponder from Joe Bloggs.
  2. Later that day I want to send another 'mass' email. I set an Email Status filter as 'not sent' within the past 7 days.
  3. Joe Bloggs is not filtered out and remains in my results...so would receive another mass email
This seems a strange way of working and not obvious - why would a reply interfere with a 'sent' status?

I have replied and asked, "How can I exclude everyone who has been sent an email within past X days? (ie. even if they have replied, I need to exclude them)"

However I'm still waiting for a response...so thought I'd post here incase anyone could assist, and also share the information incase anyone has experienced similar 'not sent' filter inconsistencies.