Is Someone Reading My E-mails? How secure are BT online services?

Hi All
I have BT broadband connection and I use it to access my e-mail via outlook
express and Yahoo's web site, as BT seems to work in partnership with Yahoo,
although knowone seems to be able to explain to me in lemon's terms why you
pay your bills to BT, but when you have a problem, you need to contact BT
Yahoo. I disovered last week that when I logged on, my inbox was full of
delivery failure messages - ok, we all get some spam, but to my surprise,
these messages appeared to have been read, even though I didn't read them.
My bulk folder had plentry of unread messages. I contacted BT about this
issue, who advised me that I could change the password to minimise security
risk, they even told me how to do it, except that the website produced the
following message:
Internet Explorer cannot display the webpage
https://register.btinternet.com/cgi-..._pass_whatever
Most likely causes:

You are not connected to the Internet.

The website is encountering problems.

There might be a typing error in the address.

This problem can be caused by a variety of issues...

I was then transferred by BT from pillar to post, spent nearly 2 hours on
the phone and noone had any explanation to what's happening. Interestingly,
this only started just after I contacted BT and informed them that I am
changing my broadband provider and they gave me MAC code. I have contacted
BT and registered my concerns over the phone, in writting and via their web
site, but still have not had a response. What should I do?
Cheers, George