You add these accounts to his profile as shared mailboxes and this increases the size of OST files (say 40GB) on his computer which can be easily corrupted and takes time to reconfigure and download the emails. Let's say a user has access to more than 5 shared mailboxes in his outlook 2019. If people are logging into those accounts directly, then they are user mailboxes and should be converted to shared, and assigned rights as Alex suggested. No authentication to the maim mailbox, no access to the shared.

Since you don't log into a shared mailbox, it's protected by the users MFA. Shared mailbox does not have primary login, therefore I don't believe you will be able to do it.Ībsolutely correct. Apparently you can use multiple phones too just follow this. You could for instance try scanning the bar code used for built in 365 MFA to add it to multiple phones. In some cases you can also set up multiple devices for authentication. All of our 365 admin accounts for clients are MFA, we use a VOIP phone that accepts SMS and sends the SMS messages as an email to a distribution group used by the team,

Now that I've said that to actually answer your question you would have to set up some sort of shared Authentication like SMS to a shared line or email code to a shared email account. That of course doesn't answer the question but would prevent this question in the first place and of course is the best practice.

Even if you are licensing them as full mailboxes for some feature you can't get in a shared mailbox the access to the mailbox should be granted through permissions on the mailbox, Full Rights and SendAs Rights. First if these are shared mailbox why are they able to be logged into directly.

Office 365 enable mfa