John,

You say the logs on usa.net looked good - do you mean they show the missing emails were passed to you?

I haven't studied all the messages so you may have tried this - brute force isolation, changing everything at once then working backwards.

Perhaps create a new temporary account from a different email client, one you don't use now. Access it from a different (borrowed?) computer, preferably with a different OS/browser and from a different location and through a different broadband provider (for example at a friends house who uses AT&T, Comcast, etc.). For a clean break, perhaps even create a new and temporary account on SMC. It may not be practical to change all these but the more the better for debugging. Temporarily switch SMC to that email.

If this it works, debugging should be straight forward by the process of elimination, switching things back towards your current configuration one at a time. At some point it should stop working again which may provide a clue of where in the chain the emails are being eaten.

Disclosure: I don't use the email feature in SMC but my experience is with general debugging. The brute force method is time-consuming but usually pretty effective.

JKJ