We restrict the sending and receiving to prevent spammers and botnets. Unlike most other smaller providers we actually watch our sending patterns and have systems in place to prevent abuse. By limiting i/o we prioritize good reputation over selling to the most customers. We also prevent senders from sending out messages that are against the policies of most other mail servers and get feedback from those servers on potential spam.
What is your strategy for protecting your SMTP server reputation? Do you have your own ASN?