I too had to switch clients to more reliable and versatile MUA than Outlook. We used Thunderbird, which has built-in support for CalDAV and CardDAV. Plus the Quicktext Addon, which saved them lots of time!
In my case, it seems that Outlook (Classic) has some serious issues with IMAP connections (no matter to where Gmail, Mailcow, etc). Especially if the user has a lot of IMAP folders.
Last but not least, as esackbauer already mentioned, with Outlook users needs to pay special attention not to get transfered to the New Outlook that steals all the user information!