Are you sure you got the message "Email sent Successfully"?
 
Because sometimes you get a message "Email NOT Sent".
 
If you weren't paying attention and just pressed Enter when the dialog came up, that's what happens.
 
This happens to us regularly, but not frequently. Maybe once in a hundred emails.
 
The other thing we occasionally get is a message about parenthesis not being balanced, and it doesn't send. But that is much more obvious, because the dialog box is about 4 times the size. The "Email NOT Sent" one is the same size as the Successful one.

If the emails are actually being sent (have you checked with recipients to be sure they are getting them?) the only thing I can think of is that your mail server is rejecting them or putting them into junk mail or something like that.