550 authentication required - email bounce

My case is about to have external MX for some mailboxes on MS Office 365 and some other generic mailboxes on the DA server.

With your latest reply I only see 3 posts here in this thread.

Most likely you still have a different cause as I don't see you saying you have the same as in post #8, so most likely a different solution is required too.

Anyway, you can have external and internal, but not on the same domain.
So for example you can use MS Office 365 on for example [email protected] but then you can't use your DA mailbox for [email protected] he must be at MS Office 365 then too.
However, if there is a domain example.com then for example.com you can use generic mailboxes on DA again.

For Office 365 on the MX page, you need to deselect the "use this server to handle my email" and then use the selection option on that page to choose the Office 365 settings.
Doublecheck the DNS settings to be sure.

For using the generic mailboxes on DA, the MX records need to point to the domain on the DA box and have the checkbox to have this server handle the mail selected.

If everything is well except for 1 domain, then we need part of logfiles to see what is happening and even best is to have the domain name.
Als we need to know if you are using local or external dns (like cloudflare).

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