Hi @David Puttock ,
Welcome to our forum!
Did you add your iCloud account as IMAP account like this?
If so, as I know, to obtain emails from multiple devices, we need to select the "leave a copy of each message on the server" option.
By the way, some contents of Icould that outlook for Mac does not support synchronization at the beginning of the article. Please remember to check it.
Hope the above help!
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