421 4.4.2 Connection dropped due to ConnectionReset

Question

Hello all,

this is our mail flow problem, any help/advise is appreciated:

We have Exchange Online with Mailboxes, On Premise Exchange 2016 and 2010 with Mailboxes, Two Connectors on Exchange Online, we can send mail from Online mailbox to on premise mailbox and vice versa.

We do want to send mail from online mailbox to external addresses while routing the mail through our on premise system.

Mail flow for online to on premise comes through and gets logged in firewall, mail scan and on premise. For online to external this isnt logged at all.

On the Online connector I´ve set the RouteAllMailViaOnPremises parameter

When sending the mail only from O365 without routing via on premises it gets through but of course spf problems and we dont want to change the setup to only use O365, also we dont want to change mx entry

I´ve tried checking accepted remote ips for our on premise setup, certificate problems(which now the Online to on premise connector should ignore), I´ve also tried stopping scanning from our firewall, in case this modifies the packages, but since it routes through for on premise mailboxes this shouldnt be the error

This is the error I´m getting on message trace in Online when sending to external mail:

Reason: [{LED=451 4.4.400 Error communicating with frontend host or destination host. -> 421 4.4.2 Connection dropped due to ConnectionReset};

{MSG=};{FQDN=....};

{IP=....};{LRT=3/21/2023 12:32:03 PM}]. OutboundProxyTargetIP: ..... OutboundProxyTargetHostName: ....

Best Regards

Answer 1

Since SMTP logging is enabled on the internet send connector, please check the log files to see if you can get more information.

The default path should be: C:\Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\Hub\ProtocolLog\SmtpSend

I suggest creating a new send connector for specific domains and setting ForceHELO to true to see if it can eliminate the problem.

You may also need to restart the Microsoft Exchange Transport service.