Question about Outlook Express

I am not getting all of my emails. Especially the ones I send and the person hit's "reply" to respond to my email.
A relative sends me emails and I do not respond and she wants to know why and I tell her that I did not receive it.
This has been going on for over a year, so what's happening?  I've contacted my internet service provider and they say
there is nothing wrong with the service, so it must be the program.
My internet Outlook does not let me go to other pages and this has been happening for a long time too.
I use Mozilla Firefox and it works great. But what's wrong with Explorer??