traffic email security.cloud

Hi all,

I have one question made by a customer of mine for which I cant find documentation to properly answer.

If Im using email security.cloud and for some reason the exchange server goes offline what will happen with all the inbound mails?
Will it get lost or will it wait on queue until the exchange server comes back up?

Also if there's some documentation regarding this any link will be much appreciated

Regards,
Roy M