the blanks you are seing in the database could because those ones are NULL
change the where clause:
WHERE emailColumn <> ''
to check for NULL as well:
WHERE emailColumn <> '' AND emailColumn <> NULL
are the invalid email addresses in your database in a valid email format?
if the address is not a valid email address format, it will not be sent to the smtp server in order for a bounce back message to occur.
in other words if the address in the database does not contain a name followed by the @ symbol and a domain Universal Email will ignore that address.