When you try to attach files to an email message in Microsoft Outlook Web App (OWA), you receive an error message that resembles the following:
The total size of attachments that can be attached is <11 MB>. Please try attaching fewer items.
Note The actual maximum size that’s listed in the error message depends on the transport send limit.
This behavior is by design.
Message transcoding between mail systems may cause message sizes to increase. When an email is sent from an Exchange 2013 mailbox through Outlook Web App, the allowed message size is 25 percent less than the maximum send limit. This helps prevent delivery failure if the message size increases because of transcoding.