Exchange 2010 Troubleshoot Inbound Connections Limits

I'm troubleshooting an issue with a particular sender sending mass mailings to a distribution list in our company.  They advise they are getting rejected because our mail server is not responding in a timely fashion.  This is causing the mail to not get through on our primary MX, which tries our secondary MX, and ultimately fails (because our secondary mx is not accepting smtp purposely)

The sender is not getting an error from the primary MX, only the secondary because of the failure.

I am wondering though if there is some rate limiting that could be the issue here.  I've confirmed through our anti-spam logs and whitelisted all sender IPS, but don't even see it reaching our Edge Server logs.

I'm wondering what is a way to troubleshoot to see if the message is not being delivered because of it reaching a certain limit on our Edge servers.

I've checked the Transport server settings, and they are the defaults, and i've reviewed the event log for any MSExchangeTransport events, and I don't see any 1021 or 1009 events.  

Are there any other event numbers I should be reviewing or some way see the SMTP traffic and hopefully capture why the edge server is rejecting the traffic?

Also is there a way to see the current/active or historical inbound connections, so I can gauge whether our limits are close to being reached. We are a large organization with heavy mail flow, so I wouldn't be surprised if this is necessary, I'd just like to see where we are now.

Thanks in advance.
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fireguy1125Connect With a Mentor Author Commented:
Enabling SMTP Receive Logging showed that the tarpitting feature on the default receive connector was causing the mass email to be dropped.  Creating a another receive connector with the source IP of the sender email server and disabling tarpitting resolved this issue.
Patrick BogersDatacenter platform engineer LindowsCommented:

Distribution groups in Exchange are for internal use only. I am saying when a mail directed to your distribution group reaches the exchange server the e-mailadress is not bound to anyone which makes exchange NDR it (if configured).

To avoid this you could create a (for example) mail-enabled massmailing user and forward all his mail to your internal distribution list.
fireguy1125Author Commented:
My mistake - the recipient is not a distribution list, but rather a User Mailbox
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Patrick BogersDatacenter platform engineer LindowsCommented:
Hold on, (for my understanding) you have one internal user which has build a local distribution list to whom he/she wants to send mail?

If so, how many addresses are there in this list?
and second, why not build distribution group(s) for this purpose?

If i am off please explain more
fireguy1125Author Commented:
No distribution list is involved - ignore that.

A particular User Mailbox/E-mail account in Exchange is not receiving e-mails from a particular sender. Where can I see the SMTP logs showing why this sender's e-mails are not passing through our Edge servers? Where can I see what my current inbound connections are to see if I need to raise the default thresholds/settings on the Transport Servers
fireguy1125Author Commented:
Resolved on own
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