Ok, I deleted the email that was partially synced in Outlook but that does not fix the problem because the server deletes the email anyway. No biggie this time because I know what's happening and I backed up my email.

I did more testing by dropping the client side networking while Outlook is synchronising a 2MB email with a pdf attachment.

Outlook settings is the following:
Server Port: 80
Use Secure Connection not ticked
Out of three tests, I always get a partial sync and email attachment is unreadable.


Outlook settings is the following:
Server Port: 443
Use Secure Connection ticked
Out of three tests, I didn't get a partial sync.

Obviously my solution is to use port 443 and secure connection.