MX records use this server among users mailing to each other

So let's say this is my situation:
2 users (userA and userB) in DirectAdmin.
userA uses external mail in form of hosted Exchange (hosted elsewhere)
userB uses mail in DirectAdmin.

Question:
How does option under MX Records ‘Use this server to handle my emails. If not, change the MX record and uncheck this option’ work exactly?

userA has this option turned off, because he uses Exchange. userB has the option turned ON. When you set a forwarder for [email protected] to [email protected], does it then get delivered to Exchange?

How does option under MX Records ‘Use this server to handle my emails. If not, change the MX record and uncheck this option’ work exactly?

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1.) Unchecking that option takes care of the fact that your server does not handle mail for user A anymore.
2.) Changing the MX record for that domain to the external server, points all mail send to that external server, which then handles can handle the mail for user A.

Yes, mail from [email protected] to [email protected] will be send through the external mail server, that's why the MX record is needed, that mail from the internet (even from your server) knows where to send mail for user A to.

I don't ever set or change the actual MX records in DA itself, since I use an external DNS server.

Why I ask this is, that DA/Exim goes looking for local users and deliver there. Just not when that option I am talking about is checked OFF, then it looks for DNS right.

That is correct, if you uncheck the local mail server it won't check for local delivery either.

If you use an external DNS server it works the same. Your server/vps will only look at the MX adresses for domains for where the mail has to be delivered, so there won't be any local delivery.
The only local delivery will be system messages to root if any.

Ofice365 & DA

Now here's another question about this topic:

The MX server of domain.com is Office 365 and has been configured in my providers DNS portal. In Office 365 I have one mailbox; [email protected]. I have set a rule in Office 365 to forward all mail for [email protected] and [email protected] from Office 365 to my DirectAdmin server. On my DA server I created the mailboxes [email protected] and [email protected]. Now here's the problem with various settings:

With the "Use this server to handle my emails." checked:
1) Mail from [email protected] to [email protected] has been delivered successfully.
2) Mail from [email protected] to [email protected] has been delivered successfully.
3) Mail from [email protected] to [email protected] has been rejected.

With the "Use this server to handle my emails." unchecked:
1) Mail from [email protected] to [email protected] has been delivered successfully.
2) Mail from [email protected] to [email protected] has been rejected.

With the Office 365 MX server in DA:
1) Mail from [email protected] to [email protected] will result in a mail loop


Does anyone know which mail setup I should use to get this all done?


Greetings, Werner

Now here's another question about this topic:

The MX server of domain.com is Office 365 and has been configured in my providers DNS portal. In Office 365 I have one mailbox; [email protected]. I've set a rule in Office 365 to forward all mail for [email protected] and [email protected] from Office 365 to my DirectAdmin server. Now here's the problem with various settings:

With the "Use this server to handle my emails." checked:
1) Mail from [email protected] to [email protected] has been delivered successfully.
2) Mail from [email protected] to [email protected] has been delivered successfully.
3) Mail from [email protected] to [email protected] has been rejected.

With the "Use this server to handle my emails." unchecked:
1) Mail from [email protected] to [email protected] has been delivered successfully.
2) Mail from [email protected] to [email protected] has been rejected.

With the Office 365 MX server in DA:
1) Mail from [email protected] to [email protected] will result in a mail loop


Does anyknow know which mail setup I should use to get this all done?


Greetings, Werner

You cannot, you can't have the mail hosted in two different providers.

I am wondering how the first option it's even working.

If Office365 is your MX server, and you send an email from Office365, it will not even try to reach the DA server, how did you set it up?