Having changed from a Sky internet account to a BT one, I've had all sorts of strange goings on over the past couple of weeks. My Sky email address still works (it's part of my TV package), and it was originally shutting me out of out my BT emails. Every time I tried to get into my BT emails via the BT homepage links, my Sky emails appeared, but no BT ones.
One BT forums site now has 75 pages of complaints about hacking and spam e mails being sent from members' addresses.
I've also suffered from this since the "switchover" to Yahoo. Someone in Estonia was sending junk emails to my contacts, from my BT internet address.