Access additional mailbox online
As long as you have permissions and know the name of the mailbox (how it appears in the address list) then this can be used for any mailbox.
Log in to Microsoft 365 at https://login.microsoftonline.com/
Once in, select Outlook.
While in Outlook, click your initials or picture (if you have one loaded) in the top right hand corner to see the following menu options. Select Open another mailbox
Start to type the name of the mailbox, e.g EU Fax
Once the mailbox has been found, click Open. It will the open this mailbox in a different tab in your browser.