Inbound mail timeout

Hi

I please need some advise...

I have a hosted Exchange 2010 installation with quite a few tenants.

Here in South Africa there is a company that sends payslips through an accounting suite, and there are some of the companies in my Exchange that need these payslips, mail flow from that domain has stopped completely, in my message tracking logs there are nothing noted.

these mails typically gets sent in batches to the companies and I suspect that I am tar pitting those mails.

from the system that these mails are sent, the SMTP error that is generated is Queue Timeout.

I make use of PRTG for my monitoring in the data center and lately I have been seeing SMTP response times on my CAS servers of round 60000ms.

Can someone please steer me in the right direction troubleshooting this issue.

All other mail is perfectly fine.

MXToolbox reports 13 ms which is also not ideal.

Regards