it is pretty much impossible for anyone to help you with such a broad question. About the only answer that makes sense is "Because something is wrong".
How do you send out the newsleter?
Do you use the same code to generate email to each service, and the only difference is the address you send to?
Do you get any kind of error with hotmail?
What happens when you forward an email from one service to another? I.e. take the gmail one that works, forward it to a hotmail account and see if it works there. Then, forward a hotmail one that doesn't work to a gmail account, and see if it works there.
good luck. If you can provide more details, you might get more specific help.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors