I believe the most likely case is that the sender did send the email but somehow it got lost in transit. The sender may be just as scared as you're. That's why he's so defensive. That's why he's eager to prove it's not his fault. He's not doing this TO YOU; rather he's doing this to clear himself.
If you convince him that neither of you is at fault, he might just calm down and both of you would work on a solution. If so, you would have turned a crisis situation into a showcase of your problem solving and leadership skills.
In the unlikely case that the sender is lying, working with him would have helped him hide a big mistake. Most people would appreciate this and if so, he would owe you a big favor down the road. And you still come out looking good to other people, like your boss.
On the other hand, fighting him would do you no good in either case. If he's proven right, you'd look suspicious. If he's proven a lier, you'd have just made yourself an new enemy.
If you convince him that neither of you is at fault, he might just calm down and both of you would work on a solution. If so, you would have turned a crisis situation into a showcase of your problem solving and leadership skills.
In the unlikely case that the sender is lying, working with him would have helped him hide a big mistake. Most people would appreciate this and if so, he would owe you a big favor down the road. And you still come out looking good to other people, like your boss.
On the other hand, fighting him would do you no good in either case. If he's proven right, you'd look suspicious. If he's proven a lier, you'd have just made yourself an new enemy.