Unable to send mails with attachments. Error in logs : "failed due to http 413"
Error on device :
"Can't Send Mail. Your message has been moved to the outbox. Please go to the outbox and try again."

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Verify the UploadReadAheadSize for  Active sync site.
https://msdn.microsoft.com/en-us/library/ms525310(v=vs.90).aspx

Ensure that it is larger than the maximum ActiveSync attachment size.

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Problem Cause

Value for "UploadReadAheadSize" .