I have several e-mail addresses with the same ISP, and only one of them do I give out to sites that require me to give one in order to access their facilities. The others I use for friends and family. When and if I start getting spam on an address, I stop using it, and only occasionally have a looksee for anything important, I don't delete the spam, and eventually the mailbox fills up and refuses to accept any more messages. By this time the odd important message will have long since ceased to arrive.
I also have an account with another ISP running at the same time, and my e-mail programme is configured to check both on the same connection.
Having an account with someone doesn't mean you have to use it, I am with BT, but I don't use any of their other facilities, especially since they tied up with Yahoo.
Giok