Ever since Exchange Server 2013 was released some users are experiencing pretty annoying mail flow issue, mostly manifested like messages stuck in Outbox or Drafts folder in Outlook or Outlook Web App.
While this issue is still not officially confirmed by Microsoft, there are, however, several solutions that can resolve it. In this post, I will present solutions known so far to resolve this. Before you start, make sure that you have latest CU installed on your Exchange Server 2013.
First, you should check if mail flow issue is maybe caused by performance issue on Exchange server. Sometime, if the Exchange server is low on system resources, it will stop some services. If you have performance issue with your Exchange, you will definitely have it recorded in Event Viewer, so make sure that you check that first.
If you are fine with available resources and performance, but still experiencing mail flow issue, try to manually restart Exchange transport services. If you are running both CAS and MBX roles on the same machine, you have to restart these three services : Microsoft Exchange Frontend Transport, Microsoft Exchange Transport Delivery and Microsoft Exchange Transport Submission. This usually helps if you experience mail flow issue after your restart your Exchange server.
DNS configuration on the Exchange Server is also pretty usual cause for mail flow issue. To make sure that you have proper DNS configuration, open Exchange Admin center, navigate to servers, and then select your Exchange server(s) and click Edit on toolbar. Now, navigate to DNS lookups, select your network adapter and manually enter the DNS server that your Exchange server should use for internal and external lookups. Most likely, it will be your local DNS.
If this doesn’t help, you can also try to prevent your Exchange transport from using DNS IPv6. However, for this, you should edit Exchange transport config files. Navigate to your Exchange installation folder, open BIN directory and find following files:
To each of these files, you should add following line:
<add key= "DnsIpv6Enabled" value = "false” />
Be aware, however, that EdgeTransport.exe.config file already has the entry but it is sent to true, so you should just change it to false.
After you do this, it is recommended to restart transport services, or the whole Exchange server.
If you have your own solution for mail flow issue, not listed here, post a comment.