I'd tell the users that their personal mail archives are only backed up the 10th of each month. So if they add mails to it, they should do the 9th of each month if they want to be able to see it at home. Until the 9th, those mails can be read from the .ost or with webmail directly from the mailserver. So if they move their mails on the 11th of the month, they can't see them at home for a month. Their choice how to manage their own archive.

A good alternative is to switch to a glass fiber connection that is 10 to 50 times faster.