I've been working on this issue on and off for over a year. It comes and goes with different users. First, for using Send Later, and then Send Forms. We have seen that customers with certain public domain names like wi.rr.com, hotmail.com, yahoo.com can't open the estimate attachment. But, when sent directly from the estimate, then they can open the attachment. So while it's a great idea AAActive, it's not the permanent solution either, but neither is mine below.  

Another way I found was to Email from the Estimate and then hit the SAVE button in Outlook, which will save it the DRAFTs folder. Then go to DRAFTS, open the email, make the changes, and then hit send.

I think there is an issue with the SEND handshake between QB and Outlook. After an email is sent in Outlook from QB, QB responds back that the email has been sent (many people may have the notice turned off). I think it's in here that the email gets changed back. 

I believe this all started with MS and QB updates. I have 30 users on 3 different versions of Outlook, it doesn't matter which one they are on. It is very random. All users have the same updates also. I don't think we should have to jump through these hoops either. Intuit needs to go back to the Desktop and fix these issues instead of focusing on their Online, Self Employed, etc. If you're on Enterprise, for a 30 user license, we pay $8000/yr.