I checked my mailgun's account's log, and I found:
Rejected: 'Errong Leng has invited you to join ACM Problem Resolving&Report' Free accounts are for test purposes only. Please upgrade or add the address to authorized recipients in Account Settings.:
Ok. I do not like to upgrade since it will require your credit card info.
I have to add the address to authorized recipients in Account Settings.
Input your recipient's address
Check your email and click "I Agree"
Confirm and clicked "Yes"
You will found the recipient verified
Ok, every thing is fine now.
Perphas I will upgrade later since you have to add each recipient one by one and wait it verified.