I am having trouble with using PHPmailer-FE_v4.0.6/PHPmailer_v5.1 PHP libraries (by Worxware) when posting form data from an internal web server to an email address on the Zimbra mail server version 6.0.5 NE. The php script works with the Zimbra SMTP server when sending to an external email address but when sending to an email address that exists on the Zimbra server the script states that the email address is invalid.
Example: firstname.lastname@example.org will reject with invalid email address while email@example.com will email properly.
The issue was related to DNS. Solved with corrected DNS.