I have 6 email accounts receiving about 3-500 messages per week. This is how I manage (using Gmail):
- I unsubscribe from mail I don't want to receive & customise my email notification settings for all online services (Trello, Facebook, Yammer, Github, etc.)
- Forward all incoming mail from all accounts to one primary account (in Gmail go to Settings -> Forwarding and add a forwarding address)
- On the primary account, I have my 'send mail as' addresses configured so I can appear to be sending mail from any of my addresses, all from within my primary account (Settings -> Accounts -> Send mail as). Note I also have 'reply from the same address to which the message was sent' setting checked.