Are they for domains you own? Are they Gmail addresses? How specifically does it work?
With the pro Yahoomail service, in addition to your email address you get unlimited (not sure how many simultaneously) alias addresses, one completely different from your own, and one entire series based on a made-up base (with your choice of add-ons, and the ability to ditch individual add-ons at will). So I have two permanent Yahoo addresses: (1) a permanent address, (2) a permanent alias, and (3-infinity) an endless stream of ditchable alias addresses with a base-name plus customizable addenda.
And of course, in addition to that, I can also forward any mail addresses from domains I own and read/write to/from those addresses as well.