If this happens then check the Application Event Log. If you see the message below then you need to change your registr settings

MAPISendMail: Failed to submit message. Attachments to the message exceeded the size limit set by your Administrator.


If there is a Key called MaxiumAttachmentSize then delete it or enter a value of 1000+. Setting it to zero appears to cause this error. 

Helpfull link: https://support.microsoft.com/en-ca/help/2813269/attachment-size-exceeds-the-allowable-limit-error-when-you-add-a-large