USPS letter mail is generally divided into First Class and Standard. Generally, Standard mail does not get forwarded if you do a change-of-address -- only First Class does.
Most of the letters that are sent with Standard postage can generally safely go straight into the recycle bin without even having to be opened.
When doing extended travel, it may be beneficial to have Standard mail discarded, and only have the First Class mail delivered into one's mailbox -- is there a way to instruct USPS for such an effect?