Cannot send email to one domain from exchange 2010

I am using exchange 2010, sending through outlook 2013.

When i send attachments to the emails leave the outbox but are stuck in the exchange queue.The user receives delated send emails.

Emails without attachments get through no problem.

Any help would be appreciated!
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When you open the queu console, look for "Last Error" column. It will show what the error is.

what attachment you try to send word,excel?
zwig2002Author Commented:
Last error: 421.4.4.2 Connection dropped due to connection reset
So when you send an email to with word attachment, it will stuck in mail queue and when you check the last error for this mail queue, it will show 421.4.4.2 Connection dropped due to connection reset?

If, yes it seem like network connectivity issue,
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The issue is likely adjusts based on the size of the attachment.
Try sending a 10k attachment, 20k, 30k, etc. to see at which point the break occurs.  The issue is,likely filtering in the remote end that makes the connection appear as idle/
The other option is check whether it might be an issue on your end scanning the outgoing attachment making the remote side terminate the connection for being idle.
Try sending the same message with an attachment to this domain and bcc to another external email address and see what info you get.
Double check to make sure you do not have specific rule applicable to this domain versus others.
zwig2002Author Commented:
I do not see any network connectivity issues. I have also recreated the smtp connector but it did not help. Any ideas on how to test this further?
zwig2002Author Commented:
I do not see any transport rules for this domain.
I can send attachments of all sizes to other domains.
Up until last week I could send attachments of all sizes to this domain as well.
We are not having any connectivity issues.
I can send to using gmail with attachments.
It is only my server that cannot send large attachments to
What is a large attachment when this issue comes up?  Check what your delivery timeouts are.
Do you have contacts at the other domain to call and check on what they are seeing?
zwig2002Author Commented:
It seems like emails larger than 7-9mb get stuck in the queue. is the largest ISP/Email provider in china. I cannot get any help from their end.
Put a rule destination 163, and attachment larger than 7mb, generate notice to sender to breakup the message into smaller portions.

Do you have a way to see what/how an email sent from gmail with a 10mb attachment received?
zwig2002Author Commented:
I rebuilt the SMTP Connector and it solved the issue.

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zwig2002Author Commented:
This was the final solution
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