Why am I not receiving some emails from certain senders?

I'm waiting on receiving an email from someone, they said they have sent it ... They've checked their sent items and it sent, so they've re sent it. I still have no received it. I tried sending myself an email, (I received my standard saved message, but not the actual sent email) ... I've sent from another account, I received that email plus I've also received a few other emails from other people.

I have checked my safe and blocked senders etc, but there is no one in there. I have checked and cleared my junk and trash, why am I all of a sudden not receiving some emails? It's rather frustrating, I've never had this problem before! Please help!