An attacker can create a loop by forging a message to your autoresponder from another autoresponder. An attacker can feed you thousands of remote addresses and let you do the work of sending a message to all of those addresses. Legitimate mail delivery can be delayed for hours or even days. Even if you don't allow relaying, your server is required to send a bounce message to the envelope sender address if a delivery attempt fails permanently. An attacker can feed you thousands of messages, listing target names as the envelope sender addresses, and let you do the work of sending bounces to those names.
Similar comments apply to autoresponders of all types.
An attacker who finds out your subscription address can forge an unsubscription request and kick you off the list. Perhaps the mailing list will send you a warning notice, but an attacker can destroy that notice in a variety of ways. An attacker can forge bounce messages. An attacker can send you e-mail that wastes your time.