Garbaged Office attatchments in Notes R5

At work we run Lotus Notes 5.0.7 clients and Notes 6 on the mail servers.

Sometimes Office attatchments (doc & xls-files) appear to be just unreadable ASCII-garbage when it is previewed in the Notes internal viewer – and is impossible to open in the associated Office programs.

Most of the times this is solved by just requesting a new mail with NEW attatchments but sometimes even multiple resends does not help.

Anyone for an explanation to what happens with these files – and a solution to the problem ?
Is the problem at our end or at the senders end ?