SMTP connection slow to close

We are runing Exchange 2010 SP3 w/Update Rollup 2 on two MB servers and 2 HT/CAS servers.  We have several application servers that route emails through Exchange and a couple of them have had issues with what appeared to be a delay in closing after sending an email.  During testing, we found that when the app is opened and email is sent, we can see the email getting delivered to the user's Outlook, but the application remains open for about 30 seconds or more.  I adjusted the -maxacknowledgementdelay to 5 seconds on the receive connectors and that "fixed" the issue for 2 of the applications.  The third one still does the same thing.  Once an email is sent, we can see that it is delivered to the user, but the application remains open for about a minute.  I changed the -maxacknowledgementdelay on this app's receive connector to 0, restarted the Transport service on both HT/CAS servers, but it still takes about a minute to close the app.  I am at a loss to understand why this one is not working as the others.  Any help is appreciated.

Thanks,
Johnny
jdholstonAsked:
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wynandkunkelCommented:
Hi Johnny,

I have a suspicion that this is less due to SMTP not closing but rather the TCP session closedown/breakdown timeout mechanism that is causing this.

Does this failure to close the connection hamper the application?
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jdholstonAuthor Commented:
Only in the fact that you can't do anything else within the application until it is done with the current email process.  Everything else continues to work fine and it is not affecting others using the application at all.
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wynandkunkelCommented:
I would take this up with the Application vendor.  This does not sound like a Network, TCP/IP or SMTP problem.
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jdholstonAuthor Commented:
That is what I was suspecting, but vendor insists that its not their app and of course my boss wants me to open a support call with Microsoft to determine how we can fix this issue.  I hate to waste the money and time only to have them tell me the same thing you and I both believe is the problem.  I guess that is what we might need to convince them it is not us...I will post an update to this when we get definitive answer.

Thanks,
Johnny
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wynandkunkelCommented:
You might be able to prove this with a manual telnet to port 25 and then send yourself and email from your mailserver.

Use wireshark or your favourite sniffer (on the server) to have a look at the traffic from the time the application connects to the time it disconnects.  This might be tricky to setup and get the capture filters right (tcp port 25 should do the trick) but then you'll have proof of how long the application "holds-on" to the connection.
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