Over the past 24 hours we have received a lot of complaints about the non-delivery of email to random recipients from random hosts on the Internet. At this point we have been able to narrow it down to Microsoft Exchange 2003 servers running IMF v2 and Microsoft Exchange 2007 servers also running IMF. In all instances the mail was acknowledged by ExchangeDefender, processed and delivered to the target mail server. In several instances mail disappeared completely. In others only certain recipients “didn’t get” the message.

We cannot stress this strongly enough: Turn OFF any SPAM filtering enabled on your servers, along with any SPF checking, validation or firewall port 25 data inspections. 

The only rules that should be enabled are those described in previous posts dealing with port 25 restrictions.

How come an email was received by one user but not on the other if they were cc’ed?

Frequent question asked today in our support portal. For example, two users in the same organization were receiving an email. Or perhaps a distribution list where a single email address delivers to Outlook/mbox and other relays externally to a Blackberry address. If one gets it and the other doesn’t thats ExchangeDefender’s fault, no?

No. ExchangeDefender does not modify/alter the message body nor does it split the message that is being delivered. The message is delivered in a single direct stream at which point Exchange further processes the message and delivers it to the delivery agent (which further relays the message or forwards it depending on local rules.) Exchange also uses technology called single instance storage, allowing a single message to be stored in the database if it is received for multiple recipients.

So, if a message was received by one user it must have been received by the other. Why can one see it while the other can’t? Turn up message tracking and see. However, we are currently seeing this as an issue related to Outlook Junk Filters and IMF, both of which need to be disabled for reliable mail delivery.