A few special care you need to ensure MailNow running well in your setup.

1. Make sure to authenticate all the SMTP setup, so that email will be sent without issue. Sending email without SMTP authentication may works if “spoof prevention” has turn off, and only for internal email among users in within the same domain.
2. Make sure to set “Allow missing “QUIT” command in authenticated session” under SMTP settings to YES. You can find this option by login with your administrator account with VO, usually avomaster, and go to Profile → Admin Tasks. *

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