If you are sending those emails using your dot mac (dot me) smtp address, Apple will only allow you to send 200 emails in any given 24 hour period. Or, (again sending thru Apple) if you sent them without a slight pause scripted into your loop, you may have overwhelmed either Mail or the Apple server.
As a test, I would try sending 10 or 20 emails then pause 20 or 30 seconds, and then continue to see if it makes a difference.
I am not using "dot me" or a script. I am using FM10's Send Mail feature (which I forget to mention) and the Email client option.
It may be that 50 emails with attachment (with embedded photo) could overwhelm an email client such as Apple Mail.
I will do some more testing.
Thank you for your comments.