Ok, so I'm seeing lots of suggestions, and whether or not you take up the advice of people depends on your use case.
Here's mine: I'm CTO of a startup, and email is an incredibly important tool. Immediate responses to key players are a must. I also get cold emails as well, which are a (much) lower priority. I get a lot of emails, in any case. Here's what I've settled on:
* My main email reading is on my laptop with MS Outlook.
* I have all my email accounts on the one outlook installation. The means I get a single reading pane for all emails, a single list of unread emails. This is a HUGE productivity boost for me. A single place to see everything, to reply to everything, and to follow up on everything.
* I configured Outlook to NOT make emails as read automatically unless I manually do so. This means that my set of unread messages are the emails that I have not seen or need to follow up on. If I do not need to follow up on an email and I want to keep it, I mark it as read and the message disappears from the unread filter list. Using the unread list as a to-do list eliminates one more list that I need to maintain. that's very important for productivity.
* I use a lot of filters. For very noisy low priority emails I send them to a separate folder where they do not appear in the above-mentioned unread list.
* Emails that I can respond to or follow up immediately, I do so right away. This gets them off the unread list and gets rid of the to-do item.
* Emails that I need to follow up on I keep on the unread list. They are very easy to find, which allows me to follow up on them quickly and not miss any items.
* I configured outlook to delay all my email sending by one minute (after I hit "send", the mail sits on the outbox for a minute before actually going out. This is a must-have feature for me because I respond to all emails quickly and succinctly. Sometimes TOO quickly and succinctly :-)
* I do receive emails on my cellphone, but that's a secondary and emergency-only email client.
I'm not the primary responder to lower priority high-volume emails (ie, support requests), and the above is what works for me when I have a few really important emails coming in a sea of emails (maybe I get ~300 emails a day).
This is what works for me, and it works REALLY well.