You can just put a first name on an envelope and it will often get to the right address and right person.
However it's not recommended to just put a first name on an envelope without the last name as it can possibly lead to issues with delivery of the envelope.
For reliable mail delivery you should always use the full name of the recipient on the envelope including their first and last name.
You should also be sure to put your address and name on the top of the envelope as a return address in the event the envelope is undeliverable.
Including the full name of the recipient of the mail ensures the mail reaches the correct person.
And in very informal situations like with close friends, using only a first name may be acceptable although for most mail, a full name is the preferred way.
While not really strict on the issue, the USPS generally does require a full legal name on mail for proper delivery of the mail.
Regardless of the type of letter you're writing, you should always put the recipient's name on the letter.
The USPS also does not accept any nicknames or initials on the envelope.
For proper delivery of mail the address of items and mail that are sent to general delivery must indicate the addressee's full legal name and the city and country of destination.
The use of just initials, simple given names, fictitious names, or figures is not permitted on articles addressed for general delivery.