An SMTP relay server is a server that is used to forward emails from one server to another. If an SMTP relay server is misconfigured, it may be possible for a hacker to use it to send spam or phishing emails. This can be done by authenticating with the server using a legitimate account or by exploiting a vulnerability in the server software. The hacker could also potentially use the server to launch denial of service attacks by sending a large number of emails at once. Additionally, the hacker may be able to gain access to sensitive information, such as the email addresses and messages of the users of the server. It is important to properly configure and secure your SMTP relay server to prevent these types of attacks.