While working on a project, I had to setup a SharePoint site on Office 365 with a Site mailbox enabled for this site.
So far, so good, this is a well known, simple and working procedure. Nothing tricky there
BUT, here it comes the issue; this site is being shared and used by external users (thanks to the external sharing feature). Here too, there is nothing tricky and it is working fine BUT the issue comes with the Site Mailbox.
As you know, to be able to use (and so access) the site mailbox from the SharePoint site, you must be defined explicitly as a site member. This is off course the case for the external users. However, external users can not access the site mailbox because of the “Guest Access isn’t supported”