Add info of email reference syntax to help debug.

htdocs/admin/mails.php

@@ -858,6 +858,13 @@ else
 		print $formmail->get_form('addfile', 'removefile');
 
 		dol_fiche_end();
+
+		// References
+		print '<span class="opacitymedium">'.$langs->trans("EMailsWillHaveMessageID").': ';
+		print dol_escape_htmltag('<timestamp.*@'.dol_getprefix('email').'>');
+		print '</span>';
+
+
 	}
 }
 

htdocs/langs/en_US/admin.lang

@@ -1971,3 +1971,4 @@ NotAPublicIp=Not a public IP
 MakeAnonymousPing=Make an anonymous Ping '+1' to the Dolibarr foundation server (done 1 time only after installation) to allow the foundation to count the number of Dolibarr installation.
 FeatureNotAvailableWithReceptionModule=Feature not available when module Reception is enabled
 EmailTemplate=Template for email
+EMailsWillHaveMessageID=Emails will have a tag 'References' matching this syntax