I use Gmail, and Gmail is really good about sorting stuff. Of course, that's not much good unless I tell it what to do.
When I get email from someone who loves my money (that is to say, a company who I've done business with in the past who I might want further contact from), I head for the "Filter messages like these" (from the "More" menu). I try different search options until I get something that looks like it'll get all of the messages of that nature from them.
Once I have that, I ask Gmail to tag all incoming messages with my "Coupons & Sales" label, optionally creating a subtag for that specific company. This way if I'm going to be shopping there, I can easily look and see if there's a current promotion before I check out. It also helps me identify what's likely to not be important when I look at my inbox.
I have a separate-but-related process for high-volume, low-importance notifications. Often I can't just dismiss the entire source, because that might blackhole things like "btw your account is on fire" or "hey, we just shipped your package". When I get those, I create a filter search that identifies that particular form of "noise" from that source (sometimes a source emits more than one kind of noise: I get a lot of email from Dreamwidth that I need to know about, but Dreamwidth's poll votes, birthday notifications, and invite code notifications are need to not clog up my day even though I want to know they're happening). Once I have the search honed, I instruct Gmail to tag all of these messages with the label "Probably dismissible". (If I feel like it, I go back and also have them automatically tagged with a label that tells me what it is, often color-coded.) (Most times when I create a "coupons & sales" search, I then go right back and put that same search in for "probably dismissible".)
A few times a day, especially if mail has stacked up, I search my inbox: "In:inbox label:Probably dismissible" which brings up everything that's been automatically tagged. I select-all for that page and look through to make sure that nothing that I actually need to take action on is in there (like if that birthday notification was for someone I need to buy a present for). I uncheck the ones I need to take action on and archive the rest of the page. Then I un-tag the ones that need action so they won't get swept up the next time I do that search.
I am still behind on my e-mail, but it's not the absolute pit it could be if I wasn't doing this.
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