Can you please help. I keep getting this error message bellow. It shows on screen for about 3-4 seconds then message that the mail was successfully sent comes on. In actual fact the mails are not being sent.

Notice: ob_clean() [ref.outcontrol]: failed to delete buffer. No buffer to delete. in .../administrator/components/com_vminvoice/helpers/vminvoice.php on line 333

Hello,
this is notice - level (lowest) error, so it is moreless informative. It should have no real impact on functionality.
Anyway, to remove it, you can comment the command on line 333. It seems to be forgotten there and has no real functionality.

Anyway, if you have problems with mails not being sent, this is most likely not the reason.
Did you check the server logs to see, whether the mails do leave your server?