So you're using a webmail app, not a client application? Which webmail app are you using? What, exactly, are you doing to try to open the document, and what, exactly, happens?
Stefano, if he's using webmail on the SME server, particularly if it is the horde suite, that could legitimately be an SME issue. Of course, if he'd said that initially rather than "a mail client" (suggesting something like Outlook/Thunderbird/etc.), that would have clarified things considerably.