Database mail failure. Exception Message: Cannot send mails to mail server. (A recipient must be specified.)

Hi, 

I have set up email notification for agent jobs. I configured database mail profile and mail account. This works fine and I can send out test email.

Database Mail Log has entries like 

Message
The mail could not be sent to the recipients because of the mail server failure. 
(Sending Mail using Account 1 (2018-09-13T13:25:16). Exception Message: Cannot send mails to mail server. (A recipient must be specified.).
)

I am running SQL server 2016 enterprise edition. It is a couple month new server, and I did not realize this failures.

I have set up same features on other servers and they are working fine.

Thanks!