add config option to reject incoming deliveries with an error during the smtp transaction

useful when a catchall is configured, and messages to some address need to be
rejected.

would have been nicer if this could be part of a ruleset. but evaluating a
ruleset requires us to have the message (so we can match on headers, etc). but
we can't reject messages to individual recipients during the DATA command in
smtp. that would reject the entire delivery attempt.

for issue #156 by ally9335

mox-/config.go

@@ -1436,6 +1436,44 @@ func prepareDynamicConfig(ctx context.Context, log mlog.Log, dynamicPath string,
 
 			checkMailboxNormf(dest.Mailbox, "destination mailbox", addDestErrorf)
 
+			if dest.SMTPError != "" {
+				if len(dest.SMTPError) > 256 {
+					addDestErrorf("smtp error must be smaller than 256 bytes")
+				}
+				for _, c := range dest.SMTPError {
+					if c < ' ' || c >= 0x7f {
+						addDestErrorf("smtp error cannot contain contain control characters (including newlines) or non-ascii")
+						break
+					}
+				}
+
+				if dest.Mailbox != "" {
+					addDestErrorf("cannot have both SMTPError and Mailbox")
+				}
+				if len(dest.Rulesets) != 0 {
+					addDestErrorf("cannot have both SMTPError and Rulesets")
+				}
+
+				t := strings.SplitN(dest.SMTPError, " ", 2)
+				switch t[0] {
+				default:
+					addDestErrorf("smtp error must be 421 or 550 (with optional message), not %q", dest.SMTPError)
+
+				case "421":
+					dest.SMTPErrorCode = smtp.C451LocalErr
+					dest.SMTPErrorSecode = smtp.SeSys3Other0
+					dest.SMTPErrorMsg = "error processing"
+				case "550":
+					dest.SMTPErrorCode = smtp.C550MailboxUnavail
+					dest.SMTPErrorSecode = smtp.SeAddr1UnknownDestMailbox1
+					dest.SMTPErrorMsg = "no such user(s)"
+				}
+				if len(t) > 1 {
+					dest.SMTPErrorMsg = strings.TrimSpace(t[1])
+				}
+				acc.Destinations[addrName] = dest
+			}
+
 			for i, rs := range dest.Rulesets {
 				addRulesetErrorf := func(format string, args ...any) {
 					addDestErrorf("ruleset %d: %s", i+1, fmt.Sprintf(format, args...))