Keep in mind that some people have email accounts with limits on file attachments.
• Some are set up not to accept attachments (for security reasons) and
• others have limits on the size of the attachment (often 1 megabyte).
If your ebooks have images, they will definitely be more than a megabyte.
Consider this as an alternative... set up a password protected link for the ebook and send the username and password by email to the buyer. Give them a reasonable amount of time to download it, say 28 hours or 36 hours (some people are away on weekends and might need more time). If they legitimately fail to download it in time, you can always send them a new link.
If you do it this way, there's no problem with size limitations or server attachment limitations. The user would be downloading the ebook the same way they download anything else from the net.