os mail sidebar mailbox list

Hello. I have quite a few email addresses for business purposes, and each of them shows up in the Mail sidebar individually.

As here is an "All" mailbox at the top of the sidebar (where supposedly all incoming/outgoing mail should be visible) would it be possible to hide all the other mailboxes? This isn't a huge thing, but it's distracting and keeping procrastination at bay is a never ending war.

Also, should synchronization happen without me triggering the process? When I realized that it wasn't happening I selected the option and immediately 4,720 messages started to download from the cloud to my laptop. All of which I had seen before. What is considered good practice and how can I keep things synchronized to avoid this happening again?

Many thanks for your help.

rlg