If it's exactly the same file, mailservers (at least some) are clever enough not to store it for each instance. New features in ZFS are also, apparently, clever

If it's exactly the same file, mailservers (at least some) are clever enough not to store it for each instance. New features in ZFS are also, apparently, clever enough not to store two files in two places. But yes, shared volumes or Google Docs is probably the way to go.