Exlcuding data from SQL table in query

I may have been working too many hours lately, or I'm simply losing my mind, but I'm having a heck of a time with a very strange result from a query

I have a simple query that retrieves emails from a list

SELECT emaillist_email

FROM emaillist

Now let's say the above gives 50,000 records

Now take this

SELECT emaillist_block_email

FROM emaillist_block

Say that gives 5,000 records

Now put them together

SELECT emaillist_email

FROM emaillist

WHERE emaillist_email NOT IN

(

SELECT emaillist_block_email

FROM emaillist_block

)

Now unless I am losing it, I should get 45,000 records, presuming that the 5,000 are in both tables.

The issue is, I get zero records.

Any ideas?

*** I Have solved the problem after digging a little deeper. There was a NULL record in the data which had been imported from XLS causing it to throw the query out, once I removed it the correct records were returned ***