A few special cares you need to ensure MailNow to run well in your setup.

1. Make sure to authenticate all the SMTP setup, so that emails will be sent without any issue. Sending emails without SMTP authentication may work if “spoof prevention” is turned off, and only for internal email among users of the same domain.
2. Make sure to set “Allow missing “QUIT” command in authenticated session” under the SMTP settings to YES. You can find this option by logging in to your VO as administrator account, and go to [Profile] → [Admin] → [SMTP Settings].

* MailNow seems to terminate SMTP session without sending a proper “QUIT” command, which in many cases, can be treated as incomplete conversation and can be ignored by the recipient server.
We have reported such issue to MailNow in August 2010. We hope that they can rectify the issue in their future releases.