As to what I keep, usually notifications about upcoming social or business-related events, some personal e-mails from friends, recipes, pictures or family news. If family or friends have changed electronic or physical addresses, I keep that. I keep financial info like Amazon or Ebay transaction confirmations for about 3 months, at least until I'm sure the product has been recieved and is in good condition. I usually keep recurring payment notifications (mortgage, energy bill, bank statement) a few months.

i try to go through on a weekly basis and delete stuff I don't need. I also try to use the folders system in whatever e-mail account I am using, to try to organize and better keep track of what e-mails I've got. I have both Yahoo and Gmail accounts and they each have plusses and minuses when it comes to folders.

I try to go through and delete e-mails that are not important, or that I have already read. I use my e-mail acct to get Facebook responses and replies, so I get 10-50 of those a day.

In contrast, my Mister keeps pretty much everything that goes into his inbox, except for flat-out Spam.

At work, I keep all my emails, both sent and received. I have found that past emails play an important role when responsibility and liability come into play. "Who said what and when did they say it," can be a determining factor in many lines of work. The ability to say, "I said this on this day, at this time and here is the email to prove it" is very important for me.

I only keep confirmation emails for when I sign up for a website, so I know what email I used to sign up for what websites (for password retrieval). Everything else is deleted - spam, finished email strings, emails I haven't gotten around to replying to, etc.

I save the password info from messageboards and sites I register for. I also temporarily keep e-mail with pertinent information. Like, if I have to print something, I have that email starred to look at later.
With gmail, I have labels and google labs features which are helpful in categorizing what's of most importance to me.

I keep nearly all personal business emails, anything ranging from my mystery shopping payments, to which sites I'm with, to banking.
I have an 'Anything school' folder.
I have a 'Tips' folders, for being frugal.
I also have a 'Recipes' folder.
Reminders or things that I won't remember.

Here's a biggy:
Personal emails from friends about updates in their lives. I rarely keep junk/spam sent from friends. You know the ones that say:
"If you send this to 100 people in the next 10 minutes your wish will come true 5 minutes after."
OR
"If you don't send this, you will die in a horrible car crash." - Funny I stopped sending those, AND I ain't dead yet...knock on wood...have to find wood to knock on ... be right back! hehehe
BTW why would friends do that to you anyways? ... that should be a mahalo question :)

Okay, that's pretty generalised as a question. By just saying "e-mail" we have categories. The Inbox, the Spam, the Trash.

Actually, I keep all. I don't go deleting the mail from the inbox, even if I won't read it or it is eligible to be trashed. With gb's of data storage in the current day technology, I have kinda spoiled myself.

I only remove spam mail. Now, it kinda keeps grabbing my attention for some reason or the other. I don't know why! Every time I see the spam box count on the left hand side in Gmail, which goes like '''Spam (5)''', I cringe. And I get straight on deleting them. Oh and for that matter, I've never understood messages in the Spam folder that start like they are real messages. You know, the ones with "Dear Mary, Thank you for...."blah. I can only see a bit of them without opening them and I must say I've been intrigued, though I've never opened them. Anyway, to come back to the topic, they find their way to the trash box.

And then there's the Trash box. Nope, I almost never empty it. What more, on deleting, you have Google popping up a message something on the lines of "Hey, where's the need to delete when there's so much space that keeps adding up continually!" Yeah, there. Blame Google. Not me! As is, it gets deleted after 30 days on its own.

So, to conclude, I keep everything. You know, I am greedy in that way. Who knows what tiny etsy bitsy of info comes to my usage someday? Yeah, yeah I know, don't roll your eyes. As a statistic, I have 13,500 mail (unread) in my inbox at the moment.

Hope I answered your question ;)

Best,
BrownEyed.