If you have a gmail account here's a little known trick: let's says your address is [email protected] You can add the + symbol and additional text after "someflyer" but before the @ sign to make each address you sign up with unique. For instance, use:

[email protected] for Spirit
[email protected] for FT
[email protected] for United's Mileage Plus

etc...

Then if you get spam sent to one of those unique addresses, you know who either sold you out or got hacked. Do bear in mind however that spammers are far from stupid. Some do prune email addresses of the +whatever stuff before sending.