There is a known issue with attempting to email directly from AccountEdge to Mac Mail on Mac OS X 10.11 El Capitan. The PDF of the invoice or purchase, etc. will not be attached to the email when sending directly from AccountEdge. This only occurs in Mail and does not occur when sending emails through Outlook.
Update AccountEdge to fix this issue
The following AccountEdge versions eliminate this issue:
AccountEdge Pro/Network Edition v15 (and newer)
AccountEdge Basic v4 (and newer)
If you're using an older AccountEdge version and have a current support subscription, you're entitled to an update. If you're not prompted to update when you open AccountEdge, go to the AccountEdge menu and choose Check for updates.