I've been having this problem. I've even tried just sending mobi files as a gmail attachment. This has always worked in the past but now it fails as well.
I've found that editing the meta data in the mobi file and removing all "calibre" references prior to sending the attachment is successful.
I used this program to edit the mobi file:
http://jarrin.net/MOBIeditor
The only conclusion I can come to is that Amazon is blocking files based on the meta-data. To restore the email functionality to Calibre the format conversion needs to stop adding "calibre" to the meta data or at least make it optional.
Cheers.