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Outbound Smarthost Connection Limits

    Question

  • Hello,

    I am seeing an issue where our primary SMTP internet send connector is routing through a smarthost (actually 8 in the list; sub1.domain.com, sub2.domain.com...sub8.domain.com) and we are getting outbound mail that queues up very high within this queue in Exchange.

    From what I can tell, Exchange never seems to attempt to open more than 2-3 connections for emails ready in this queue at a time.  I have checked the max outbound connections and max outbound connection per domain.  These were 1000 and 20 respectively and I have tried setting them to unlimited.

    The issue seems to be that when these messages go through the smarthost, the smarthost holds the connection until it either fails or delivers the message.  So with a mistyped domain in an email for example, the connection breaks after 60 seconds with a temporary 4.2.1 failure.  It then goes through the process of later retrying.  All the while, more mail stacks up, and so on.  Eventually we get to a point where regular mail is taking 10-15 minutes to deliver outbound.

    If I change to deliver mail directly using MX records, all of the mail jumps out of the queue in no time, which makes sense, as each outbound domain gets its own queue essentially and the failures just stack up on themselves rather than in a single queue.

    What I am wondering, is why can't I get Exchange to open 50 connections to the smarthosts at a time?  Or what would be causing it to only ever try 2-3 at most at a time?  I have tweaked some of the settings like the timeouts of the messages, increasing the retry intervals, changing the Queue Glitch values, etc.

    Anyone else experienced this or have recommendations on how to use outbound smarthosts but not have poorly spelled domains stacking up the queue and slowing down regular delivery?

    Thanks in advance!

    Monday, March 20, 2017 6:27 PM

All replies

  • Hello,

    I am seeing an issue where our primary SMTP internet send connector is routing through a smarthost (actually 8 in the list; sub1.domain.com, sub2.domain.com...sub8.domain.com) and we are getting outbound mail that queues up very high within this queue in Exchange.

    From what I can tell, Exchange never seems to attempt to open more than 2-3 connections for emails ready in this queue at a time.  I have checked the max outbound connections and max outbound connection per domain.  These were 1000 and 20 respectively and I have tried setting them to unlimited.

    The issue seems to be that when these messages go through the smarthost, the smarthost holds the connection until it either fails or delivers the message.  So with a mistyped domain in an email for example, the connection breaks after 60 seconds with a temporary 4.2.1 failure.  It then goes through the process of later retrying.  All the while, more mail stacks up, and so on.  Eventually we get to a point where regular mail is taking 10-15 minutes to deliver outbound.

    If I change to deliver mail directly using MX records, all of the mail jumps out of the queue in no time, which makes sense, as each outbound domain gets its own queue essentially and the failures just stack up on themselves rather than in a single queue.

    What I am wondering, is why can't I get Exchange to open 50 connections to the smarthosts at a time?  Or what would be causing it to only ever try 2-3 at most at a time?  I have tweaked some of the settings like the timeouts of the messages, increasing the retry intervals, changing the Queue Glitch values, etc.

    Anyone else experienced this or have recommendations on how to use outbound smarthosts but not have poorly spelled domains stacking up the queue and slowing down regular delivery?

    Thanks in advance!


    I would look at what the smarthost allows for connections.

    Exchange 2007 reaches end of life on April 11th. What’s your plan to move?

    Monday, March 20, 2017 6:34 PM
    Moderator
  • Hi Andy,

    We are using Barracuda Email Security Service, and their support confirmed they support up to 3000 connections per smarthost.  So we should be able to connect much faster than what exchange is connecting at.  But not sure how to debug or further prove that out.

    I have been able to do SMTP logging and can see the connection timeouts when connected but not the attempts to connect to the smarthosts at a higher rate than what we see in the active queue.

    Tuesday, March 21, 2017 12:32 AM