Change "httpRuntime maxRequestLength" in the web.config file (in Exchange server) from to 15 MB or above.
 
Reason for increase of attribute value : For any message size limit, you need to set a value that's larger than the actual size you want enforced. This accounts for the Base64 encoding of attachments and other binary data. Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.
Ref :- https://technet.microsoft.com/en-us/library/hh529949(v=exchg.160).aspx