Have your considered placing the users mail store on a Coda Files System (http://www.coda.cs.cmu.edu/) or similar? CODA supports disconnected operation and multiple write masters. I used it for a distributed CVS archive. The only problem we had was that during network outages between sites it was possible to have the same file modified in both write servers thus requiring an administrator to resolve the conflict (see Coda manual for details on this) when synchronizing. Considering that Courier uses unique file names for mail storage you shouldn't have this problem. Also, since each unique file will most likely only be accessed by one user at a single site, the chances are very slim that the file could be modified on both sites while the link was down.

You might want to check out Coda. I have thought about it a little in the case where we end up locating our servers at multiple data centers. Since that is still a ways out I haven't tested this in a lab environment yet with Courier.

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