email client that is able to handle calendar invites with multiple email/calendar accounts

alright so I'm posting here a guide for how to get this all working in Outlook, in case anyone else needs it:

1. Set up a different calendar for each email account, by deleting all Exchange accounts (restart Outlook) + then set each IMAP account to be the default email account + also the default data account (that’s the next tab in accounts, that’s important because otherwise it does not work). Restart outlook after doing so for each outlook account. After doing this for each account, re-add the Exchange account(s) back in.
2. Then connect IMAP calendars with rhe CalDavsynchronizer plugin for Outlook. In Cpanel, you can find the setup settings by going to “manage” for an email address.
3. If you want to check if the appointments are being synced: the calendars are only visible if you use the Horde email client, not if you use Roundcube.
4. After setting up a different calendar for each IMAP account, you can accept meeting requests directly in the email. The reply email address will now be the correct one.
5. Setting up a meeting appointment yourself has to be done by:
   1. From the email tab in Outlook, and then “new”, and then meeting
   2. From an email from someone, and then hit the 3 horizontal dots at the top right of the email, and then ‘meeting’
   3. Or by selecting the specific calendar you want in the calendar tab, then making a new appointment there. If you don’t do that first, and just click ‘new meeting’ in the Calendar tab without first selecting the right calendar, the meeting will be sent from the default exchange account, even when you select a different email address as the sender.