Outlook 2010 introduces a new limitation: a limit on the number of files that can be attached to a message at once from a network share. File size is not the problem: you will get this dialog attaching 21 1KB files or 21 1MB files.
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 by this feature (or bug, depending on your point of view). However, there is another issue that affects attachments on a local drive. See Using a Local Drive (below) for details.
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. Outlook 2010 has this limitation in an effort to reduce network overhead.
Using Network Drive
When attaching files to an email message and the files are on a network drive, you are limited to selecting 20 files at a time. If you select more than 20, you'll receive an error message:
"You are attempting to attach too many files at once. Try reducing the number of files being attached."
Using a Local Drive
You may receive this error when you are attaching files from the local drive if you used the Search field in the Insert File dialog. I haven't yet identified the criteria that triggers this error as you can search and attach files found in different folders. Some users say more than 36 files triggers this dialog but I triggered it with as few as 25.
Solution: Select fewer files at a time or attach files from one folder at a time. To avoid the error, try selecting no more than 30 at a time when you use Search to find the files.