I ran into this case this past winter, as we tried to grind out the MS Exchange migration. The objective was simple: Move all mail, calendar objects, tasks, memos, etc.. from Outlook PST archives to the Exchange server. This would enable our users to access their mail from Outlook, OWA (Outlook Web Access for the non-MS initiated), and any other device, application or whatever that can talk to an Exchange server (not many of them, BTW).
As I proceeded down my checklist per each user, I would come across a very FULL trash bin. I would empty its contents, compress the PST, and in doing so, speed up client and reduce the amount of disk space required. Win-win!
A week later I'm restoring a PST file (or two) from backup because someone "filed their stuff in the 'Deleted Items' folder".
Its 'Deleted Items'. Trash. Garbage. It gets emptied.
I carefully explained to the user that the trash folder is the first place we'll use to recover space, should we need to. We won't; we've got about 2tb of space for mail, and we're currently using about 80gb. So there's room to grow.
But that's besides the point. You make file folders in a cabinet to store your bills, you don't file them in the trash bin.
When you're at home, do you put left-over dinner in the garbage expecting it to be there tomorrow?
I sure don't. I mean, I guess you could.. but...
So, a few months pass, come around to recent times, and someone I know is working in Google Mail.
Gmail, as you probably know, is just a different beast. It doesn't work along the same lines as a typical mail client, with folders for sorting and whatnot. Gmail uses labels. So?
Well, if I am interested in web development, I might have a label for web development, PHP and CSS. An email may pertain to one or many of those labels. In the end, if I want to see all my emails relating to Web Dev, I can pick that label - and I will see all those emails. Same with CSS and PHP. Now if I want to see all stuff that is Web Dev and PHP, I can make some filters or a custom search query to get what I want to know.
But I digress. The issue that this person had was removing the emails from the inbox. To solve the problem, this person moved the emails to the trash.
Then, wondered why they disappeared.
"But.. it should tell you that!"
But.. it does. It's trash. It's meant to go byebye when the garbage man comes by every month.
"But.. how do I get it out of the inbox?"
Well, you archive it. Yeah, that's not plainly clear, but one of the things Gmail touts is "Never need to delete another message".
See that top message, from Kendra? That's in my Inbox. See the second message from Kendra? The one that's starred? That's not in my inbox. But I still have the email message, and the system I use means that that message requires my attention. That's another post.
Gmail, for me, provides a great amount of flexibility for me with this method. Emails that are retrieved from POP accounts get tagged with that account. That helps me identify who's emailing what address, in the event I close any of the "main" accounts. I've been having problems with my web-host, and with excess load, and one of the symptoms is that my Drupal cron jobs generate a 404 error (instead of an HTTP 200 code). So, create a filter to meet the criteria that I want, and any cron message that doesn't have a 200 return code in the message gets tagged red, and left and in inbox. Otherwise, it gets "archived" and I never see it (until I want to clean it up).
Messages need my attention, but darnit its sleepy time and much better handled while I'm at work? Star it, and archive it. This whacks it from my inbox, but keeps the email.