Potato-potato. Proposition or not, it exists and it is used. Your customers want to use it. You could implement it like described or implement it completely in a proprietary way. You could lead and help getting it standardized or you could wait at the tail-end until somebody does that for you - loosing customers on the way. Either way - the feature is needed by your customers not some obscure standard. You are the developers - you decide how to do it. I trust you can make the best decision.
Do you plan on implementing this feature? If not, can you please elaborate more as to why not? If there is support for Exchange, etc. surely you can invest some time in implementing this as well.