Although unaware of the exact memo system you are referring to, @money-dreamer, my thought is that the charge could be made high enough to discourage all spam, provided a configuration setting on the receiving end could specify the percentage of the charge to be deducted from the standard fee. This could range from all to nothing, with sender given that info just prior to sending the message.
There was a simular debate during my involvement with a Devcoin Bounty Project, which offered email options to SMF Software.
P.S. Also a list of those one would except no charge memos.