check/dns: Mark require_matching_echo as deprecated

See #280.

docs/man/maddy-filters.5.scd

@@ -55,14 +55,6 @@ Action to take when check fails. See Check actions for details.
 Log both sucessfull and unsucessfull check executions instead of just
 unsucessfull.
 
-## require_matching_ehlo
-
-Check that source server hostname (from EHLO/HELO command) resolves to source
-server IP.
-
-By default, quarantines messages coming from servers with mismatched
-EHLO hostname, use 'fail_action' directive to change that.
-
 ## require_mx_record
 
 Check that domain in MAIL FROM command does have a MX record and none of them

internal/check/dns/dns.go

@@ -158,6 +158,8 @@ func requireMXRecord(ctx check.StatelessCheckContext, mailFrom string) module.Ch
 }
 
 func requireMatchingEHLO(ctx check.StatelessCheckContext) module.CheckResult {
+	ctx.Logger.Printf("require_matching_echo is deprecated and will be removed in the next release")
+
 	if ctx.MsgMeta.Conn == nil {
 		ctx.Logger.Printf("locally-generated message, skipping")
 		return module.CheckResult{}