Member-only cybersecurity website CyberWarrior.Academy posted an article about developing a vanilla PowerShell reverse shell that uses SMTP communication to fly under the radar.
The article described how data can be embedded in SMTP headers and status codes, and while the maximum total length of a text line is generally 1000 octets, this can be increased via SMTP Service Extensions.
The article then presented a proof of concept, demonstrating a functional reverse shell created in this way and stressed that the article was written for educational purposes and was only intended for legal penetration testing and red teaming activities where permission has been granted.