As far as I know, you cannot simply disable Junk-Email in Outlook for cloud based mailboxes. This setting will not be applied. You can instruct your users to add false positives to Safe Senders and Domains List in Outlook or in Outlook on the Web. You as administrator can also add certain domains and senders in the users own lists using Powershell Set-MailboxJunkEmailConfiguration
Please find the description here: https://docs.microsoft.com/de-de/powershell/module/exchange/set-mailboxjunkemailconfiguration?
Another approach would be to create custom Junk-Email Filtering policy and apply it to a certain users, groups or domains. You can also modify Anti-spam inbound policy (Default) to white-list certain senders and domains. We have, for example, more that 120 in allowed domains list
