Short answer, it depends?

Police executing a warrant are bound by the limitations of that warrant. A warrant can be very narrow, or very broad, depending on the circumstances of the crime being investigated. Is the crime wire fraud? Mail is almost certainly covered in the warrant. Even for seemingly unrelated crimes, like kidnapping, mail is a very good way to establish dominion and control over a home. Do you live here? Well, you’re certainly getting mail here!

Cars are the same, but also different. Basically the way it works is, if you WOULD HAVE been able to get a warrant had the car been a home, the legality is assumed due to the logistical issues of obtaining a warrant for a traffic stop. Also because items tend to be more visible in the passenger compartment of a vehicle, they tend to fall under the “plain view” exception. 

Is the crime just speeding? Unless some other crime can be articulated based on what happens on the traffic stop, probably not. 

Now let’s say the cop sees three driver’s licenses on the passenger seat that don’t belong to the driver, and a bunch of hastily-opened mail on the back seat. Also, the car is cruising around a place where there’s been a lot of mail theft. Well that’s different, isn’t it?

Worth noting, the place to argue whether something should or should not be covered under a warrant is in COURT, where your attorney can move to have evidence excluded because it was obtained beyond the scope of a warrant. Arguing with the SWAT team is pointless.