Using the attention line correctly when addressing an envelope is more than good business-writing etiquette. It helps guarantee that your mail is delivered to your intended recipient.
Write the "attention" line first. This is written with the abbreviation ATTN. Note that there is no punctuation mark with the abbreviation. The attention line always precedes the name of the company. Example: ATTN JOE SMITH
Write the name of the company or organization where your recipient is located. Example: ATTN JOE SMITH ABC COMPANY INC.
Write the street address, including any suite numbers, on the next line. Example: ATTN JOE SMITH ABC COMPANY INC. 123 MAIN STREET STE 789
The last line of the address should be the city, state and ZIP code. Example: ATTN JOE SMITH ABC COMPANY INC. 123 MAIN STREET STE 789 WASHINGTON, DC 12345-6789
TL;DR (Too Long; Didn't Read)
The U.S. Postal Service requests that addresses are written entirely in uppercase with no punctuation.