Hi @Cesar ,
From your description, seems like what you are encountering is related to the proxyshell vulnerability as mentioned in the blog below:
ProxyShell vulnerabilities and your Exchange Server
According to the blog, if you have installed the May 2021 security updates or the July 2021 security updates on your Exchange servers, then you are protected from these vulnerabilities. So for your situation, it's suggested to install the latest CU and SU on all your Exchange servers to protect your Exchange environment against these threats.
Furthermore, here's a thread which discuss a similar issue for your reference:
Unexpected Spam email in Outlook Draft folder
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