Consult it before you send any poem out. And maintain the list rigorously. This is one of those unbreakable, mostly unbendable rules. SASE stands for Self Addressed Stamped Envelope . And every submission you send out has to have one.
Editors are usually very busy, invariably under-paid, people.
Okay, and some of them are lazy. When they've read your submission and decided it isn't right for them, they want to be able to immediately stuff it in an envelope you've supplied to them, with your name and address already filled in, and send it back to you. They're also fairly cheap, so the envelope better have postage already applied. Some writers like to print off a dozen or so envelopes with their address, then keep them in a drawer for future use. Others, like myself, prefer to print two envelopes for every submission.