It's strange that your sims would be reset while getting mail in any vacation world. Does the same sim always get the mail, or do multiple sims get reset? If it's the latter, I wonder if there's some stuck object that the game is trying and failing to deliver.
The first, and easiest, intervention is always to delete the five cache files in your game folder in Documents. They fill up with stale data and should be cleared regularly even in the absence of game issues; while you're troubleshooting, delete them every time you quit. I'm not sure this will help, but it certainly won't hurt. For reference, the files are:
- CASPartCache.package
- compositorCache.package
- scriptCache.package
- simCompositorCache.package
- socialCache.package
It should also be noted that there's no reason your sims
have to get their mail every day. If they own a vacation home, you'll get an alert when their bills are past due and can pay by clicking on the button in the alert message. It's also best practice to save, quit, delete the cache files, and reload immediately before your sims travel, including when they return home. So you could pick up the mail after you reload, immediately before you end their vacation.
Granted, that's not a solution, just a convenient workaround. I'm not sure how to actually fix this without mods. In the homeworld, you could go to Edit Town, evict your sims, save their house to the bin, bulldoze the lot, replace the house, and move the sims back in. If you did it all in one Edit Town session, without saving the sims themselves to the bin and replacing them, they'd keep their jobs and relationships but lose their promised wishes and opportunities. That's still more invasive than you might want, but I'm not sure it would work at all in a vacation world. If you want to try it, be sure to save immediately before, then use "save as" afterwards to rename the save, so if something goes wrong later, you can revert to the prior save with no harm done.
With mods, specifically NRaas MasterController, if this is a case of a broken item that can't be delivered, you could run a reset of the entire world. Click on the base camp's map tag (it stands in for City Hall in a vacation world), and select NRaas > MC > Town Options > Reset Everything. This will also reset your own sims, unless you've disabled it (Settings > Include Active Lot in Reset Everything > false).
You might also be able to flush the object from the world using MC. If you click on the ground out in the world, i.e. not on any lot, and select NRaas > MC > Object Stats > Non-Inventory. If you're not sure what the object might be, you can select Object Info > Non-Inventory from the same menu; each object you check will get its own report in the upper right corner. Clicking on the thumbnail of the object (or right-clicking; I never remember which) will focus on the object, but an object that's stuck undelivered will have no location, and the camera will zoom out to a bird's eye view. This also happens for objects on sale at various shops (like the consignment store at home), but it's not going to matter too much if you flush those as well.
Anyway, I'm sorry that there's no easy solution, at least none that I can think of, but then it's an odd problem to have.
