550 5.1.10 RESOLVER.ADR.RecipientNotFound

After adding a domain to O365 tenant, getting a bounce back when emailing from domain already in O365. So @domaina.com is already in O365 and we are planning to setup a Hybrid Exchange 2010 with domainb, so when domainb is added to O365 and a user@domaina.com emails him he gets a bounce back. We have not setup the Hybrid yet as the server still needs to be patched to SP3. I tried setting domainb to Internal Relay in O365 so it would look for the MX records but still get the same error. Do I need to setup a connector with the Exchange server?

Original Message Details
Created Date:      3/6/2018 7:32:24 PM
Sender Address:      user@domaina.com

Recipient Address:      user@domainb.com

Subject:      Test


Error Details
Reported error:      550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient user@domainb.com not found by SMTP address lookup
DSN generated by:      DM5PR19MB0954.namprd19.prod.outlook.com


Thanks
abhijitm00Asked:
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timgreen7077Exchange EngineerCommented:
I'm not clear. Are both domains already in O365 or just domaina.com?
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abhijitm00Author Commented:
Domaina is already in O365 with MX record pointing to O365. Domainb was added in preparation for Azure AD Connect and Hybrid setup. Domainb has MX record pointing to Barracuda appliance
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timgreen7077Exchange EngineerCommented:
ok since both domains are in o365, o365 looks at itself and can't find a user or mailbox for the recipient even though it can see the domain in o365. so domaina, gets the bounce back because there most likely is no mailbox or user for domainb in o365. You will need to create a send connector in o365 and say anything going to domainb go out of that send connector. this will allow the email to leave o365 and look at the mx record of domainb and be delivered to your on-prem mailbox until your hybrid is setup.
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timgreen7077Exchange EngineerCommented:
just for clarification on the send connector, when you create it instead of using * you will use doaminb.com
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abhijitm00Author Commented:
Got it I will try that
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timgreen7077Exchange EngineerCommented:
good luck.
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abhijitm00Author Commented:
Hi Tim, I am getting the following error when trying to setup a connector to on-prem Exchange server:

Reason: [{LED=450 4.4.316 Connection refused [Message=Socket error code 10061] [LastAttemptedServerName=1.1.1.1] [LastAttemptedIP=1.1.1.1] [CY1NAM02FT007.eop-nam02.prod.protection.outlook.com]};{MSG=Socket error code 10061};{FQDN=1.1.1.1};{IP=1.1.1.1};{LRT=3/6/2018 9:38:38 PM}]. OutboundProxyTargetIP: 1.1.1.1.

Would this have something to do with firewall? We have not restricted smtp to any particular IP range and it is set to any as of now

Thanks
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timgreen7077Exchange EngineerCommented:
did you use a smart host or anything when you setup the new connector. If so don't use a smart host in o365. can you send me the settings you set on the new connector?
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abhijitm00Author Commented:
Yes I specified the IP address of the on premise Exchange server
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timgreen7077Exchange EngineerCommented:
Ok, disable the connector you created. When you are in o365 looking at the connectors, do you see any connectors that are enabled or are there no connectors.
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abhijitm00Author Commented:
There are no connectors and I could not create a connector without specifying a smart host or an IP
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timgreen7077Exchange EngineerCommented:
where does the MX record of your on-prem domain (domainb) point to? Does it point to your exchange servers or does it point to the barracuda and the barracuda send the email to your Exchange.
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abhijitm00Author Commented:
Barracuda and that send the email to Exchange server
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timgreen7077Exchange EngineerCommented:
Ok, point the connector to the barracuda public IP address not the Exchange server.
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timgreen7077Exchange EngineerCommented:
It should allow you to validate also the make sure it connects.
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abhijitm00Author Commented:
Thanks I will try that
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abhijitm00Author Commented:
Hi Tim, I was able to setup a connector to on-prem Exchange server and the email flow started working. I was trying to add an incorrect IP address. Thanks for your assistance
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timgreen7077Exchange EngineerCommented:
Excellent. Take care and good luck on your O365 setup.
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