If nobody has access to his mailbox, then whoever sent it had to either have his login info and password, or he left a device he'd logged in on accessible. Either way, you are responsible for securing your account, so whether he sent it or not is moot, really.
Is he claiming he wasn't even on the network at the time the message was sent? It should be easy enough to check the event log on his machine and see if he was logged in, or vpn'd in, or whatever, depending on whether or not it is expedient to find out if he is the kind of liar who over-elaborates his lie in the hopes of making it more convincing. But it's probably not a good idea if he's management.
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Better educated sadness than oblivious joy.
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