What RFC does SMTP Prevent needs 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.

See as reference the documentation Symantec_DLP_10.0_Email_Prevent_MTA_Integration_Guide.pdf on chapter 3 (page 17).
Mime compliance is outlined in http://en.wikipedia.org/wiki/MIME#MIME-Version