I am trying to auto forward external emails from Exchange 2007 via a send connector to a smarthost, the smarthost points at a CAS server hosting Exchange 2013, which in turn proxies the mail to the mailbox server running Exchange 2013.
Emails from gmail, hotmail, outlook.com and other domains are being delivered into the Exchange 2007 mailbox, being auto forwarded to a mail contact, which sends across the send connector and are being delivered to the appropriate mailbox on the Exchange 2013 server.
Emails sent from one domain in particular is failing with the following NDR SMTP error
#5.3.0 smtp;530 5.3.0 Too many related errors> #SMTP#
Does anyone have any idea as to what could be causing this error?
The queue on the Exchange 2007 server is showing the following error when trying to send email from this domain
Target IP address responded with: "421 4.4.2 Connection dropped." Attempted failover to alternate host, but did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.
As I recall, I have tried disabling AV and increasing the max protocol errors on the receive connectors on both Exchange 2013 CAS and Mailbox server, but neither have made any difference.
Wireshark trace shows 3 delivery attempts, but it eventually fails with the 5.3.0 error
Thanks for your help