If you've sent a mass email with GMass by copying and pasting addresses into the To field, and fewer emails were sent than you expected, then the reason is likely that your list of email addresses was tainted with extraneous characters.

For example, if you paste in 500 email addresses into the Gmail Compose To field, put in your Subject and Message, hit the GMass button, and then find that only 300 emails were sent, you should examine what you placed in the To field.

You'll likely find that a large portion of the email addresses were surrounded by a pair of double-quotes. When that happens, everything inside the double-quotes is treated as one single email address. Those are likely the email addresses that did not receive the email and make up the difference between the expected count and the actual sent count.