If the email address in the CSV file has a trailing (i.e. invisible) space, the email address is deemed invalid by Moodle when processing. A space (before or after) can easily be added inadvertently when cut and pasting from other files.

Please add a "space delete in email field" step to the processing of the CSV file.
It is a minor bug, but annoying.
I am finding this in version 2.4