DNS and E-mail configuration recommendation for Subdomain ListServ setup in existing Exchange Hybrd environment?

Ah ok. So we have MX record already that point to our Mimecast gateway. Mimecast  inbound then hands off to our on-prem Exchange Server.

So we'd need to configure on the Exchange Server an Accepted Domain for "listserv.ourdomain.com", and then create a Send Connector on the Organization Configuration > "Hub Transport" section with "listserv.ourdomain.com" in the Address space, then either the IP or FQDN for the "Route mail through the following smart hosts" section?

Then on the Sympa server, just set the relay to the same Exchange server?

And no changes needed on our public DNS?

You have to add an mx for listserv.ourdomain.com pointing to your existing mx.

You can double check by
Dig MX listserv.ourdomain.com @8.8.8.8.

If results=0 then you have to add MX otherwise email does not get routed

About the configuration you described, woth the limits of a forum consulence, it seems to me correct if you have a standard configuration in receive connectors

Ah ok, so could t just use an A record in this case.
By returning 0 with the “dog” Test, , do you mean the Answer flag value?

*dig

Ok both - i would use an mx because its clearer, though

If you got anymore questions please ask

Ok sorry I didn't get much sleep -- so here's the plan:

As for the SMTP flow, I’m thinking:

MX Record added for list.ourdomain.com to deliver to Mimecast.
list.ourdomain.com added as acceptable domain in Mimecast.
Exchange Send Connector is setup for list.ourdomain.com to send through the Sympa server (as a designated smarthost for list.ourdomain.com).
list.ourdomain.com added as acceptable domain in Exchange.
Sympa processes/relay its e-mails through Exchange server Receive Connector, which goes out our normal/external (*SMTP) Mimecast Send Connector.  

That seem right?

Also, would we need an SSL certificate added to Exchange for list.ourdomain.com?

No certificate is needed.
Mail flow for listserv will be relayed thru mimecast to exchange. Exchange will accept it as it has the corresponding accepteddomain. How exchange will route email to listserv? It needs a send connector scoped to the listserv domain and with smarthost field pointing to listserv ip

As for receiveconnector, it needs a receiveconnector which accepts listserv emails on port 25 and which does not need listserv to authenticate

Thank you so much

Ver ySorry I had one follow-up question. So if the Sympa server relays e-mailto @list.ourdomain.com through our on-prem Exchange server, and the Exchange server has a Send Connector to the Sympa smarthost for e-mail to @list.ourdomain.com, will that cause an e-mail loop? Or will it go out through the internet to SMTP * send connector?

Sorry do not understand

Ver ySorry I had one follow-up question. So if the Sympa server relays e-mailto @list.ourdomain.com

Is this list.ourdomain.com another 3rd lvl domain coming into play? Usually listsservers are used for distribution lists and are setup to deliver mail to their own domain locally.

It’s an on prem list serv that would be email addresses lime @list.ourdomain.com but it’s distribution list members would be a mix of ourdomain.com Exchange recipients as well as external members.

Thanks much for your help and patience

hm sorry one more question, I see get an error when I check message queue, any idea what I should try and change?

Identity: Mail9\Submission\16296490
Subject: Test e-mail
Internet Message ID: <DM6PR@DM6PR.namprd08.prod.outlook.com>
From Address: sender@domain.com
Status: Retry
Size (KB): 15
Message Source Name: SMTP:ReceiveFromMimecast
Source IP: 188.154.77.31
SCL: 0
Date Received: 8/16/2018 9:53:00 AM
Expiration Time: 8/18/2018 9:53:00 AM
Last Error: A local loop was detected.
Queue ID: Mail9\Submission
Recipients:  test@list.domain.com;2;0;;0;

You are routing messages to @lists. to your external smarthost. Double check that you Put the ip of your listserv in the connector scoped to the listserv itself.