Everyone NEEDS rate limiters. Every email server should have rate limiters as part of the SMTPD process. Both IP rate limiters for the inbound attackers, and even more important.... OUTBOUND per user rate limiters. Especially if you operate a large scale email server for email hosting, ISP's or Telco's. Spammers are busy using keyboard sniffers, trojans, and brute force attacks to get your customers email passwords, and when they do.. Look Out! The will spam like crazy from that account, get you on all kinds of blacklists, and cause all kinds of other problems.
If your email server does not have rate limiters, call our sales department and we can help you out.