"Warning: Failed to send mail to ..." message when Distribute via email

When doing a reload task with "distribute via email" enabled, the task finishes with warning.
Tasklog shows some warning like this:
(2014-05-15 11:21:33) Warning: Failed to send mail to name@company.com
(2014-05-15 11:21:33) Warning: Distribution reported warnings for resource

But test emails can be successfully sent out in QMC > Mail server.

The test email is sent by Management Service, whilst distribute via email is done by Distribution Service.
There is possibility that:

• Management Service and Distribution Service are running under different account with different permissions;
• Management Service and Distribution Service are running on different servers with different permissions.

That is why permissions on both services (and servers) need to be checked by mail server administrator.

There are several points to check in order to troubleshoot this issue.

1. Successfully sending test email indicates email account authentication setup is correct alread. Nothing else needs to be done on QlikView side;
2. Make sure the service account that runs Distribution Service has permission to access email server and send out emails;
3. Make sure the server that runs Distribution Service has permission to access email server.
4. Both points above need to be checked by email server administrator.