Dear all,

I have a question

Let's say I have two servers in DAG environment and all databases are collected on both servers if something happens with the 1st server and it's no longer available I understand that the 2nd one will be responsible for sending/receiving e-mails

How it will affect on clients Outlook? Do they need to restart Outlook or it will be connected automatically?

Thank you in advance

Outlook client will connect automatically if you have some mecanism to switch between each server (Load Balancer with proper healthcheck, DNS round robin, ...). So the response is neither in Exchange nor Outlook but in what you get between them.

Outlook -> HA solution for Client Access -> Exchange

You don't mention it in your question, but both of the MBX servers in your DAG would also have to have the CAS role installed too, if those are your only  two Exchange servers.  What I mean is, if for some weird reason only one of the two servers in your DAG was multi-role (both MBX and CAS roles installed) and that's the server that failed, your clients would have no CAS server to connect to.  Now, assuming both of your servers are multi-role, then what Bruce JDC said is correct.  If you have a standalone CAS server(s), then you have nothing to worry about; the CAS server(s) will automatically send client communications to the MBX server holding the active copies of your DBs.