What do those check marks mean?

These symbols indicate the different status of your letters.

• Airplane: Your letter is currently in transit and on its way to the recipient.
• Single grey tick: Your letter has been successfully delivered to the recipient.\
• Double grey and yellow ticks: The recipient has read your letter. 

It is important to be aware that if someone removes you from their friend list, your letter will still show a single grey tick, indicating that it has been delivered but not read. The recipient won't be able to read your letter unless they choose to undo the removal action, at which point the status will change to double grey and yellow ticks once they read it.