When sending reports per email, recipients are only receiving winmail.dat files :They have two kind of problems.
The problem is:E-mails created by Axapta use the Outlook rich text encoding, which is not supported by many e-mail clients.It would be very difficult to make Axapta send e-mails in different format. It uses the SMAPI interface to send report e-mails, and this API doesn't have any means of enforcing any given encoding. To be able to do that, a completely different API would need to be used (for example MAPI, using the workaround described in KB278321), and that means that core e-mail logic would need to be rewritten - a change that would probably need much work, and has potential of breaking many things in Axapta.
Please go through following KBs on Exchange configuration to resolve the problem: