In the world of hosting, the spam police might show up at your virtual door with a DNS blacklist with your IP address on it. If you truly have no idea why, there is a good chance that a spammer has found a way to exploit your server or VPS, and an open relay could be the culprit.
An SMTP server is the part of your mail server that handles sending. Any outgoing mail has to be processed by the SMTP server. Ideally, that server has some security measures in place that keep unknown users from exploiting it. An open relay is an SMTP server that allows anyone to use it. Spammers can use it to send mail, and it will appear as though it came from you, even if you have never actually sent anything.
DNSBL (blacklists) will identify open relays and inform other mail servers to deny them. As a result, if your server ends up on a blacklist, you may not be able to send mail to other mail servers that use a blacklisting service. Once you fix the open relay, the blacklists will eventually remove you, but it may take some time.
Open relays are easy to fix with proper SMTP authentication, and you can find online tools that will test your mail server to see if it has an open relay. With that in mind, there is no reason to delay fixing this potentially dangerous security issue.