Users of network file shares should note that Outlook 2010 has a limit of 20 files when selecting multiple files from a network drive. When the user selects more than 20 files, Outlook 2010 responds with an alert:
“You are attempting to attach too many files at once. Try reducing the number of fields being attached”
Users can attach more than 20 files that are stored on a network share, however they need to select the files in groups of 20 or less. Files stored on your local drive are not affected, only files selected from a network share trigger this alert.
Although Outlook 2010 doesn’t limit the number of attachments users can attach to a message, it will prevent users from adding attachments that cause the message to exceed the size allowed by the Exchange server, or 20 MB, if using other account types. This setting can be changed, see Outlook 2010′s Default Attachment Size for more information.