A quick note about file size. The data size of a email message doesn't just include your attachment, but also the content of the message, and perhaps, that picture of you on vacation in your signature :o)
Let's say that you are sending a file that is an image, and on your computer that file is 5MB. An image file can not send via smtp. It must be converted to something that can send via smtp. That converted file can be larger that the 5MB you started at with.
Second, what do your delivery logs say? Is SM creating a new message and a new message ID? Look at the logs. SM has a fantastic logging system, and at minimum you should have deliver, smtp, pop and imap logs set to detailed.
Finally, get to your spool right away, and clear these messages before you get multiple emails sent.
Now to the issue. I have come across a similar issue, and it is most likey your AV soft ware on the machine that is sending or a corruption of the PST file. You do not mention the program you are sending from, but I bet it is outlook 10, or 13, and my money is on 2013. Google will show articles on fixing a corrupted PST file, but ultimately, have a good backup, delete, and recreate. The attachment most likely has nothing to do with your issue.
Remember kids, every time a spam message gets blocked, a nerd gets their glasses. spamhurts/July 15