Send mass email function

Hi I have over 1500 contacts on Zoho with emails attached to them. We sent a mass email this morning to 'all contacts' and we got an email saying that it only sent to 400 people? (See attached image).

But then, when I went onto zoho and into that particular campaign, it says it sent to 921! (Which is still not ideal as we have a lot more than that in our database but I do appreciate people unsub etc.) Does anyone know how I can add ALL contacts to my next campaign or where they are pulling the above numbers from? Thanks!