Did they use the correct reply to address? Check.
Their outbox clear? Check.
They got no 'message failed' notification? Check.
Junk mail/deleted folders checked? Check.
Blocked senders checked? Check.
Have we got them to send a test to the generic email? Check. That didn't get through either.
Have we checked our servers for any sign whatsoever of these mails? Check. Nothing.
Other people having sent the same thing with the same attachment successfully? Check.
At this point I have decided that someone has cut the pipe and is siphoning off all the bits to increase their bitcoin stash.
And yes, if it were only one person I'd say they're bullshitting about having sent it, but it is more than one, I spent 15 mins on the phone with one yesterday as he tried to send it as we spoke, and still nothing. It seems completely random but because it's computers I know it isn't. Geeks - why would this happen?