Legally, you are generally allowed to write on letters that you receive, as long as you are not engaging in any illegal activities such as tampering with mail or committing fraud. However, there are some important considerations:
Privacy Laws: It's crucial to respect privacy laws. Writing on someone else's mail could potentially violate privacy laws, especially if the information you add or disclose is sensitive or private.
Tampering: Tampering with mail is a federal offense in many countries. While writing on a letter might not be considered tampering, you should avoid actions that could be perceived as malicious or fraudulent.
Consent: If you plan to write on a letter and send it back out, ensure that you have the consent of the original recipient. Unauthorized alteration or use of someone else's mail could lead to legal consequences.
Intent: Your intent matters. If your actions are meant to harass, deceive, or cause harm, they may be considered illegal or fall under harassment laws.
In summary, while writing on letters might be legal in some cases, it's important to consider the context, respect privacy laws, and ensure that your actions are not malicious or fraudulent. If you have any doubts or concerns, it's advisable to consult legal counsel for advice specific to your jurisdiction.