An SMTP server configured in such a way that it allows anyone on the Internet to send email through it, not just mail destined for or originating from known users. This is not a recommended configuration because it can be exploited by spammers. Servers with open relays are routinely blocked and/or blacklisted.
To get more information please check out our Email Infrastructure Guide
If you have a question that needs an answer, please contact support. Otherwise, please open an issue in our GitHub! Thanks for helping us improve our docs!