[exim] what's my exim up???

Dear all,
When i send a email to many account in a domain !!!
Ex : I sent to a@??? b@???,c@??? ...
But only a@??? & c@??? received my mail , b@??? is
not!!! I looking for the logfile and i see record save a ,b and c
received email!!!The
I don't know why b@??? don't received my email!anh where's this
mail store??!! Please , help me!!!
Thanks,