Calendar invites getting accidently routed to Shared mailbox in Outlook Office 365

Hi ,

I have a client running Outlook 2016 on Office 365 - He has multiple shared mailboxes setup. He is getting an odd phenonmena where calendar invites, outside or inside his organisation are going to his shared mailbox (called help@...) as opposed to his personal one (simon@) which people are emailing to- Any subsequent replies to that invite continue going into the help box as well.
There are no rules running on the client end. What could be causing this?

Thanks
D