Not everything. Some items cannot be mailed, for legal and safety reasons. Some items are mailable but only in limited quantities with special packaging. Fraudulent information and pornographic materials are not mailable.
It is the mailer’s responsibility to ensure that they do not mail anything that can injure postal employees, postal equipment, or customers’ mail.
Refer to these documents for complete information on:
Hazardous, Restricted, and Perishable Mail
Written and Graphic Matter
Some mailpieces are too small to be mailed -- they are likely to get lost or jammed in postal equipment. Some mailpieces are too large to be mailed -- they are simply too big for the Postal Service to handle. Some things are not the right shape to be mailed.