Mass email success rates?

Mass email success rates?

Hi All, I have used Z Recruit for 6-years, and now have 6 Consultants working on it.
I've raised this query a number of times over the years, but we are really concerned that a number of our mass email campaigns do not actually ever get delivered / seen.
We run a number of campaigns per day, and generally most are under 100-Contacts in a single mail-out. Sometimes we do not ever get any replies or even 'Out of offices' (I know Z recruit has the 'Sent' and 'Opened' status but these seem very limited.
If we re-run the same campaign via Outlook, we generally get a much better response rate...does anyone else suffer with this, and is there anyway of knowing / proving if the Z Mass email campaigns are being marked as SPAM (we have verified our email domain).
Any help would be great.