SMTP MITM refers to a malicious actor setting up an SMTP proxy server between the UniFi Controller version <= 5.10.21 and their actual SMTP server to record their SMTP credentials for malicious use later.
https://hackerone.com/reports/519582
https://community.ui.com/releases/9f698d0b-8279-40d3-9f1a-d36db4813124
https://community.ui.com/releases/862b962b-55f6-4324-96be-610f647d5c1c