A possible format would be zip for instance. There is however another option for Outlook users. It is possible to add a key to the registry to enable certain file types that are then no longer blocked in Microsoft Outlook. To do so hit Windows R, type in regedit and hit enter.

Navigate to the key HKEY_CURRENT_USER / Software / Microsoft / Office / 10.0 / Outlook / Security for Outlook XP. Change 10.0 with 11.0 if you run Outlook 2003. Right-click in the right window and select New > Key. Name it ‘Level1Remove’ (without the ”) and add one or more file extensions to the key. It could for instance look like ‘.exe;.vbs’.

This would unblock .exe and .vbs file types in Microsoft Outlook.