Just like in this: 

https://answers.microsoft.com/en-us/windowslive/forum/all/attachments/2908a724-3a64-431f-825b-0960974c8957

I have the same problem. I send emails but the attachments - that I KNOW are there - don't send. This happens when the file sizes are on the large size (today's was about 4.4MB) or when I send several at one time. It's not consistent. Sometimes I get lucky.

I use Windows 10, the built-in Microsoft Mail program with an outlook.com mail account. (This forum forced me to pick "Live" mail but I don't think it's technically called that. It's just called "Mail.")

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The problem does not happen when I use a browser to send the email. I can also get the email to send the attachments if I send a NEW email... instead of a reply to an existing email thread. It's very odd. 

This is affecting my ability to do my work and deliver things to my clients. Please help. Thanks.