This is indeed one of the more controversial changes which were introduced with Outlook 2010. You usually get this error when trying to attach more than 20 files at once from a network share. When you try to add them from your local hard disk, you won’t receive this error.
It’s a bit of a half-successful attempt to refrain people from attaching many files to a message within a corporate network (where it is more common to work with network drives).
Although the limit isn’t configurable or can be turned off, there are many workaround for this nuance.
A “Yes, thank you, attach anyway” button might have been more user friendly.
(click on image to enlarge)
Workaround 1: Send shortcuts to the files
If you are indeed in a network where the recipient can also access the network share, consider sending shortcuts to the files or the directory containing the files instead.
To quickly insert many shortcuts to files at once see; Adding hyperlinks to files in bulk
Workaround 2: Attach the files in batches
If at first you don’t succeed… try again! Once you’ve added your first 20 files, you can add another 20, and again and again…
Workaround 3: Zip the files before attaching
Instead of adding many attachment individually, another option/workaround would be to contain all the file files that you want to send in a single zip-file. Many 3rd party compression utilities have a “zip and email” functionality to do this in one go from within Explorer.
Workaround 4: Attach the files via drag & drop
My favorite method of attaching files (regardless how many) is not to use the “Attach File” method within Outlook at all but drag & drop them from Explorer into my message.
Since you usually need to browse to the files anyway, I’d much rather do that in a proper Explorer rather than via the small and limited “Insert File” dialog. You can quickly open an Explorer window via the keyboard shortcut +E
Workaround 5: Upload the files to a file sharing service
If you reconsidered and don’t want to send the files as attachments after all or when their combined size is larger than you allowed to send, then you can also use a file sharing service.
In a corporate network, you might have access to a SharePoint server for this. Ask your helpdesk if you do not know.
For personal usage, Dropbox, OneDrive (formerly known as SkyDrive) and Google Drive are popular file sharing services. You can set permissions on the files so that only the intended recipient can access them.