Problem with adding subscribers to queue

[mabezdek]

Hello,
In our last newsletter campaign we send around 13 thousend emails. Unfortunatelly secessfully has been send only about 1000 emails. Recipients: 12937 Sent: 1000 In Queue: 0.

We are looking for an reason and we found only one possible reason - limit “Add emails to queue”.
What exactly does this limit and why is set up here?

Could be reason of last incident that the person who started the campaign didn't wait to complete ajax requests to fill all subscribers to queue?

Thank you,
Martin