I ask, because I can't find any emails in Gmail from June 2008. They have all been deleted... forever, it appears.
I've heard that after 30 days in the trash, messages are no longer recoverable. Is this true?
And if so, is there a way to delete messages, but ensure that I'll be able to view those messages 4 years from now?
Archive your emails instead of deleting them. Then you can always access them with the "All mail" link. If you send your emails to the trash, they will be removed after a period of 30 days.
On a different note, if you don't trust the cloud fully, I recommend taking a backup of your email and storing it locally. Then you can always access them no matter what happens to your Google account or if you accidentally delete important emails.
One way of taking a local backup is using Mozilla Thunderbird and a plug-in that can save local copies of your emails in a standardized format. Another benefit of that is that it becomes easier to search in your collection of email (ironically, the search feature of Gmail has serious flaws).
The great benefit of Gmail is the use of labels. Every email starts with the inbox label. Then depending on filters that you setup they can have other labels added. Each message/conversation can have multiple labels, as opposed to a folder system where a message/conversation can only be in one folder.
In a small number of cases some emails will end up in spam, though the user has some control over that as well.
Once you no longer want to see a message/conversation on the front page of the system, then archive the email/conversation using the archive button. This action only strips the inbox label from the message. A message will automatically be reloaded into the inbox if a new message is added to the conversation.
A message can be easily found via searching with label:mylabel. plus the usual methods such as from, date, subject, and that it contains specific words. You can also quickly jump to all the messages with a label by clicking on the label in the list of labels on the left side of the screen.
Look at the documentation, and other questions on this site that discuss the use of filers, and multiple inboxes.
Never delete, but label and archive everything.
