Microsoft Outlook has a maximum size for adding traditional attachments, so that’s why you get this message. Dragging an attachment into an e-mail has the same effect as adding with the paperclip.
When you choose to insert the attachment via the AttachingIT plugin, you can. The attachment isn’t processed in the mail. Therefore, always opt for larger attachments to add the attachment directly via the AttachingIT plugin.
It can manage with small attachments. When you enter an attachment in the traditional way, the item is first linked to the mail. Then the attachment is extracted from there.