You need to leave the 'Enable DNS lookups' box checked as it's needed for mail delivery. You need to look in the logs and see what the error is that causes the mail to be deferred, it will tell you why it can't deliver it.
You can check your DNS with the following commands, please run them on the zimbra server and post the output here:
I fully expect that to return valid results as what you're describing is an intermittent problem that (most likely) is caused by a network problem but let's see how we get on with that output.
dig yourdomain.com mx
dig yourdomain.com any
host `hostname` <-- use that exact command with backticks not single quotes
Please also post the output of the command I gave you earlier:
su - zimbra