Problems with sending to one domain

Hi

Exchange 2010 SP3.

Messages sent to the addresses of just one remote mail domain is stayed in the queue. Some messages reach the addresses, but with up to few hours delay .

In the message properties in this error queue, errors like 421 4.4.2 Connection dropped due to TimedOut

Tried to clear messages and restart the Exchange transport. After that, several messages are sent normally, and then the queue starts to grow again.

We have contacted other side and they says that they have these problems only from our addresses, and they added our server to whitelist.

But messages stuck in queue to that server anyway.

I'm confused by one thing from our side:
When message stuck in the queue - smtpsend logs looks like:
2018-03-27T16:40:34.625Z,Internet,08D593D896149536,2,10.10.5.245:17075,<remote server ip>:25,<,220 mail.company2.com ESMTP,
2018-03-27T16:40:34.625Z,Internet,08D593D896149536,3,10.10.5.245:17075,<remote server ip>:25,>,EHLO mail.company1.com,
2018-03-27T16:40:34.657Z,Internet,08D593D896149536,4,10.10.5.245:17075,<remote server ip>:25,<,250-mail.company2.com,
2018-03-27T16:40:34.657Z,Internet,08D593D896149536,5,10.10.5.245:17075,<remote server ip>:25,<,250-8BITMIME,
2018-03-27T16:40:34.657Z,Internet,08D593D896149536,6,10.10.5.245:17075,<remote server ip>:25,<,250 SIZE 104857600,
2018-03-27T16:40:34.658Z,Internet,08D593D896149536,7,10.10.5.245:17075,<remote server ip>:25,*,31497,sending message
2018-03-27T16:40:34.658Z,Internet,08D593D896149536,8,10.10.5.245:17075,<remote server ip>:25,>,MAIL FROM:<user1@company1.com> SIZE=2232024,
2018-03-27T16:40:34.690Z,Internet,08D593D896149536,9,10.10.5.245:17075,<remote server ip>:25,<,250 sender <user1@company1.com> ok,
2018-03-27T16:40:34.690Z,Internet,08D593D896149536,10,10.10.5.245:17075,<remote server ip>:25,>,RCPT TO:<user2@company2.com>,
2018-03-27T16:40:34.721Z,Internet,08D593D896149536,11,10.10.5.245:17075,<remote server ip>:25,<,250 recipient <user2@company2.com> ok,
2018-03-27T16:40:34.741Z,Internet,08D593D896149536,12,10.10.5.245:17075,<remote server ip>:25,>,DATA,
2018-03-27T16:40:34.898Z,Internet,08D593D896149536,13,10.10.5.245:17075,<remote server ip>:25,<,354 go ahead,
2018-03-27T16:50:35.557Z,Internet,08D593D896149536,14,10.10.5.245:17075,<remote server ip>:25,-,,Remote


So we have "354 go ahead response", and in 10 min connection closed.

In the same time, messages to that domain from other addresses come without any problems. As well as to other addresses from our server.
I checked our server for open relay and other problems on mxtoolbox - all looks OK!
Ivan BogachevIT engineerAsked:
Who is Participating?
 
Riaz Alexander AnsaryConnect With a Mentor Enterprise Infrastructure Systems EngineerCommented:
1- are you or the other side use any messaging gateway such as proofpoint or ironport for filtering?
2- are you authorized to use the domain you are sending emails from in your exchange organization? some emails services doesn't not accept messages that fail spf test right away and depending on their setup the delivery maybe timing out.
3- check the reputation of your domain. always avoid sending bulk emails from user mailboxes.
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Martin MillerConnect With a Mentor CTOCommented:
This may be a DNS problem on your side or theirs... and possibly linked to an MX record update.

Use command line or online DNS tools to note their MX records, then find a way to query your internal DNS, and compare.
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zc2Connect With a Mentor Commented:
Did you try to do telnet to that remote server, port 25 and manually enter the SMTP commands?
EHLO mail.company1.com
MAIL FROM:<user1@company1.com>
RCPT TO:<user2@company2.com>
DATA

Subject: test

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Ivan BogachevIT engineerAuthor Commented:
Did you try to do telnet to that remote server, port 25 and manually enter the SMTP commands?
Yes, I sent 3 test emails using telnet and all of them were delivered immediately
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zc2Connect With a Mentor Commented:
I would try the following: Save an email which can't be sent to an .eml file (Thunderbird can do that), then try to paste the content of the file after you entered the DATA command to the telnetted SMTP
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Ivan BogachevIT engineerAuthor Commented:
1- are you or the other side use any messaging gateway such as proofpoint or ironport for filtering?
We have Microsoft TMG and they have Ironport on their side.
2- are you authorized to use the domain you are sending emails from in your exchange organization? some emails services doesn't not accept messages that fail spf test right away and depending on their setup the delivery maybe timing out.
Yes, we have SPF records configured
3- check the reputation of your domain. always avoid sending bulk emails from user mailboxes.
Our reputation is OK. We are not blacklisted.
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Ivan BogachevConnect With a Mentor IT engineerAuthor Commented:
I would try the following: Save an email which can't be sent to an .eml file (Thunderbird can do that), then try to paste the content of the file after you entered the DATA command to the telnetted SMTP
I can export queue messages to eml files using Powershell. But how to paste it to telnet DATA ??
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zc2Connect With a Mentor Commented:
Just use the clipboard.
Try to send one message at a time. Once you send the DATA command press Ctrl+V
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Martin MillerCTOCommented:
It is ok to close this ?
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