We are experiencing email with attachment can't send out issue. We are on version 6.0.2. There did not seems to have such problem when we used it for last 2+ years - same platform and version. The attachment are just normal pdf (not big files) and there within file size limit. Only 2 external parties currently can't receive such emails with attachment from us. The rest is ok. The system log (/var/log/zimbra.log) shows time out message. However emails without attachment to these affected external parties were ok. Our DNS is hosted by external ISP not by us. So far no changes to that as well.
Can anyone please advice what else can we check to resolve this. Thanks.