User is in the mail (which does exist) and gets messages that seem to indicate the user's mail file does not exist.
In all causes, the issue is either missing files or permissions issues.
You go to search your mail file. This is the first time you've searched, so your Full-Text (FT) index does not exist. In Notes 8, it will create it for you, unless of course, you don't have rights to create a new folder within your data folder for the application's FT index. Instead of saying that you don't have writes to the data folder, the search wrongly assumes it cannot access/find your mail file. You most likely don't have permissions. (Or less likely, you don't have the space, which would give a similar but different error message saying space instead doesn't exist.)
Solution 1 - Option 1
Re-install the user with the Notes client only install and click the checkbox option to install for "Anyone" (to the Users/<my name> folder where user has rights) instead of just me (c:\Program Files\IBM\Lotus\Notes\data where they don't have rights), or manually give the user rights using an system administrative account. No copy their data stuff to their folder, verify permissions are correct, update the notes.ini path variables that have changed and install type info as needed. Then have user start Lotus Notes and test.
Solution 1 - Option 2
Update permissions for the user's data folder to include create folders, create files, change files, delete files, and modify files. Check for any system policy or Windows security update that will "fix" the folder and re-mess it up.
You chose to have a signature and point it to a file. You have two machines, but either forget to copy it to the second one, or the paths are different (very likely). You go to create a new mail message to send and viola, file does not exist.
If the paths are the same on both machines, simply copy the signature file to the 'other' machine. If the paths are not the same, either re-install Notes on one to the other's folder format for the data folder, or use text instead.
You see this error sometimes when you are first loading your mail file. You have a really awful remote network connection to the server. One or more frames of your mail file will display that you were unable to load content, or in a really bad case, you didn't load the main mainFS frameset and it didn't load and you got the message, file does not exist.
If you are just missing a frame, click the reload option within the 'bad' frame to reload it. You're good to go.
If you are missing the whole thing, and your network is this bad, have your administrator help you set up a local replica of your mail file, and turn on replication. Your connection sticks to bad to use as a normal "cloud" service.
You click your mail icon in bookmarks and get file does not exist.
You're mail file does not exist at that path. It is actually correct in this case. Check your location document that you are correctly pointing locally or by home mail server to your mail file.