I am using a German mail provider, gmx.net and connecting using IMAP.
When I restart Outlook or hit the send/receive button Outlook attempts to download every message again, as a result is always syncing and is rarely up to date.
Also if I delete an mail from Outlook and then purge the trash, the message is not deleted from the server.
These symptoms are the same on Outlook 2011 and 2016.