Reduce (or increase) how often you check messages
A number of people recommend checking email less often — say, once or twice a day — and leaving your email client closed the rest of the time to avoid distractions. For some temperaments, that might be exactly the right choice, and if you don't get a lot of time-sensitive email, it's worth a try.
For me, however, the best choice was the opposite — check email more frequently. If I did it just one or twice a day, the pile of messages would be overwhelming, so I keep my email client open when I'm at my desk and check my iPhone or iPad regularly when I'm not, answering or otherwise processing most messages as they come in. That way I almost never have more than a few messages in my Inbox, which improves my mental health.
Rethink Inbox management
My technique for handling incoming messages works exceptionally well for me (see "Empty your Inbox"). However, that's just one approach, and you may find that an entirely different method works better for you. As I've written elsewhere, the crucial first step is realizing that neither email in general nor your email client in particular is to blame if your email feels out of control. Reflect on what's not working well for you and experiment until you find a good technique.
To be sure, you might benefit from products or services that do some of the heavy lifting for you, such as Sanebox ($49-469 for a two-year subscription) or C-Command Software's $30 SpamSieve. A different email client or other technological changes might also help. But none of these things can compose thoughtful message replies for you or eliminate the need to think about your incoming email entirely — some parts of inbox management will always require manual effort.
That said, I can offer a few quick tips:
Use server-side filtering: Gmail, iCloud, and many other email providers let you set up rules or filters on the server that can file, delete, or even send canned responses to messages that match certain criteria. Even though your Mac email client can do that too, relying on server-side rules means you benefit from that pre-filtering even when using a mobile device.
Learn to love your Delete key: I've long had the habit of saving almost all incoming messages, but those files do take up space, and they make operations like backups, disk repair, and migrating to a new Mac more time-consuming. Plus, even with great search tools, having a larger haystack makes the needle that much more elusive. So, I'm trying to train myself to spend half a second before performing that reflexive filing keystroke to consider whether I might ever refer to this particular email message again, and if not, delete it to improve my signal-to-noise ratio ever so slightly.
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