What RFC does SMTP Prevent need to be in compliance with?
SMTP Prevent acts as an RFC-2821 compliant SMTP proxy which in general extends RFC 821.
This means the generated email has to be compliant to RFC 2821 and subsequently RFC 821.
RFC 2821: Simple Mail Transfer Protocol (rfc-editor.org)
See as reference the documentation: Configuring the Network Prevent: Block SMTP Message action (broadcom.com)