Hi,
currently i have co-exist environment for exchange 2010SP3/Exchange 2013CU8. DNS still pointing to Exchange 2010. I'm in the midst of testing mail flow where i encountered issue of sending email from exchange 2010 can't deliver to Exchange 2013 mailbox. We have no issue if email send from Exchange 2013 to Exchange 2010.
What i observed is email stuck in Exchange 2010 queue with error "451 4.4.0 Primary target IP address responded with: ""421 4.2.1 Unable to connect."" Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts"
i tried few steps like telnet 25 from exchange 2010 and it works. i'm able to send email via telnet command but user that using Outlook 2010/2013 having problem sending email to user in Exchange 2013 mailbox. (just 1 direction only)
kindly help.