We have a campaign that sent and we have a ton of emails that are not even being processed. Does anyone know what to look for?
I assume that I am replying late and the delivery you were concerned about has already finished.
You had not described as what stage it is pending and what is the status of the delivery when you observed this.
1>. If the delivery status is personalization in progress , preparation in progress etc. then the problem is likely with a busy database.
2>. If you have already gone past the phase and have started the delivery and the status is in start pending phase , there could like be an issue with MTA service or IP affinity.
3>. If the delivery is in-progress status or retry status then you need look in the delivery logs or mtachild logs to understand if there is any repeated errors.
Also , as you say you have sent tons of emails , every instance has a set capacity of email processing and you may see messages pending if that threshold is breached.