Sending calendar invite from IMAP account

Until recently, I had a bunch of email accounts connected to Outlook 2016.  It was a mess to deal with, so I moved  5 of the accounts to a different client.  I now have 3 accounts left in Outlook, 2 Office 365 accounts and an IMAP account.  Previously, calendar invites to my IMAP account would get added to my primary O365 account calendar and, if I wanted to send an invite from that account, I could create an event on the primary calendar and just change the from address. 

After removing the other accounts I lost the ability to change the from address for calendar invites to my IMAP account (or to my other O365 account for that matter).  How can I get that back?