The server must use code 211 or 214 if it accepts the request, 421, 500, 501, 502, or 504 if it rejects the request:
220 heaven.af.mil ESMTP HELP 214-SMTP servers help those who help themselves. 214 Go read https://cr.yp.to/smtp.html. HELP PLEASE 504 I don't support HELP parameters.
The server's envelope is unchanged by HELP.
There is a HELP extension that promises support for the HELP verb. Many servers support HELP but don't announce the HELP extension. Clients don't pay attention to the HELP extension; they simply try the HELP request.