After making a further investigation, I have found one thing.
For this scenario, let the zimbra domain is example.com.
If from the outlook, I am sending emails to
user1@example.com and
user2@gmail.com, where the user1 does not exist in the domain, then the mail is getting undelivered for both the users.
If I am trying to send only to
user2@gmail.com, I am able to send emails.
May be this could be the reason