When sending mailing incorrect amount of contacts displayed
I had a similar problem, probably with version 4.3, and found that a recent import had resulted in contacts with "deceased" set to null, while the query that selected e-mail addresses for a mailing excluded contacts where "deceased" was not false, thus excluding all those newly imported contacts with "deceased" set to null.

I re-imported those contacts, explicitly setting "deceased" as False, and then I was able to send e-mail to them.