We have 2 Exchange mailservers within our network that until recently had been receiving & delivering all emails. All emails go through a third party for spam filtering purposes but due to the current issues the MX records have recently been modified to by-pass this third party. The issue is when sending emails from external webmail clients, or similar, such as 1&1 emails the system does not show that the emails are being delivered to the server when we track & trace the message. Prior to the MX records being changed MessageLabs, our third party, were able to tell us that some emails returned a reference code to them as expected but others were showing no reference codes. If we utilise the same account, previously used via webmail, but utilise a client such as Outlook 2007/2010 the emails reach our users irrespective of if they are HTML or plain text messages. I have contacted one of our suppliers who use a linux based email server called Merrick which then goes through a barracuda system for spam purposes. The client systems within this company use Outlook 2000 along with some templates that are HTML based, if sent in this format they fail to show on our exchange server but plain text comes through. To add further confusion if I use a Gmail account from the internet the emails fail to come through, but using an Android based phone & sending the email it works with no issues. As mentioned we have removed all 3rd party companies from the loop, turned off all spam filtering on the server along with any services not required. The server was then updated with all patches today & rebooted... once the system came back online I was able to send an email from the Gmail web client but 6 minutes later the system failed to accept any further emails from this client. Does anyone have any ideas on what may cause this issue or things we could check to try to find out what is causing this. Lack of any logs or error messages sent the the sender/receipiant the issue has now got difficult to diagnose. Many thanks in advance for any advice anyone can provide on this issue.

Hi Clive_H Did you make a test from the external network(internet) through NSLOOKUP your SMTP address space? Then could confirm the MX changed is successfully. We could also use telnet to your exchange server port 25 from external, and then make a test. Regards! Gavin TechNet Subscriber Support in forum If you have any feedback on our support, please contact tngfb@microsoft.com Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Hi Clive_H, Any update for your issue? Regards! Gavin TechNet Subscriber Support in forum If you have any feedback on our support, please contact tngfb@microsoft.com Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.

Morning Gavin, Apologies for the delay in getting back to you regarding this issue things have been busy on the helpdesk recently. The issue was actually due to a firewall we have stopping HTML based emails with certain content types, hence once this was rectified the issue disappeared. Again many thanks for your help.