Cannot access to shared mailbox using IMAP for secondary domains

[Fab]

Hello,
I need to configure 2 shared mailboxes in a client, using IMAP. This is the scenario:

Username is: user@defaultdomain.com

Shared mailboxes are:
xxx@defaultdomain.com
yyy@additionaldomain.com

All works fine when I configure the shared mailbox xxx@defaultdomain.com. The username I set in email client is: user@defaultdomain.com/xxx@defaultdomain.com, while the email address is xxx@defaultdomain.com.

Unfortunately, I cannot access to yyy@additionaldomain.com. When I set user@defaultdomain.com/yyy@additionaldomain.com username and yyy@additionaldomain.com email address, I got login failed error.

Please note that using owa, user@defaultdomain.com can access to both shared mailboxes, so it is not a permission related issue.

I tried to create other shared mailboxes for additionaldomain.com and primarydomain.com. In all cases, user@defaultdomain.com can access (in IMAP) only to defaultdomain.com shared mailboxes.

I noticed also another strange issue: yyy@additionaldomain.com has also an alias yyy@primarydomain.com. If I delete it and save settings, when I refresh the page, the secondary address comes back again.

Does anyone had before similar issues? How can I fix them?
Thanks

[#2]

Out of curiosity, have you tried with "user@defaultdomain.com/yyy@defaultdomain.com" ?

[#3]

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@Salvatore Biscari wrote:

Out of curiosity, have you tried with "user@defaultdomain.com/yyy@defaultdomain.com" ?

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Yes... but login fails also in this case...

[#4]

An update to my case: the described issues affects only one of my two tenants. So it could be caused by some specific configuration...