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Load Balancing smtp for exchange tcp timeouts

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we have running citrix sdx with vms version 12.1 build 55 running to load balance our exchange servers. One of the configured vserver is for smtp loadbalancing. 


Our development department which sends through our erp system thousands mails per day reported since last friday that their smtp client breaks after some time. This happens if they send 10 or more mails. Other systems reported mail problems, too.


But we cant see any problems. The issue is not reproducable. 


Has someone any ideas where can i begin to search for the issue ?

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We load-balance our Exchange 2016 SMTP relay through our NetScalers.  We enabled Source IP for the Exchange servers so that they could "see" the actual IP address of the sending clients and this allows us to restrict access to only the clients we want sending through our service.


If you think your problem is due to timeouts (or lack thereof), I suppose you could temporarily disable all but one of the back-end servers handling the SMTP traffic or add a timeout to the SMTP Virtual Server if there wasn't a timeout.  Disabling all but one back-end server would eliminate the possibility of affinity/persistence being the problem, but your one server would need to be able to handle the load.


If you think your problem is due to lack of actual client IP information, you can enable Source IP on the NetScaler and back-end servers.  (If there is anti-spam software running on the back-end servers handling SMTP and all the mail appears to come from a single address, then the anti-spam software may think the SMTP servers are receiving spam.)  You could further add an exception within the anti-spam software, if present, to allow your ERP servers to send without spam inspection, but this would require actual client IP addresses, and therefore Source IP.


These are just guesses.  Let us know how you're getting along.

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