==== REQUIRED BASIC INFO OF YOUR IREDMAIL SERVER ====
- iRedMail version (check /etc/iredmail-release): 0.9.8
- Linux/BSD distribution name and version: Ubuntu 16.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): MySQL
- Web server (Apache or Nginx):Nginx
- Manage mail accounts with iRedAdmin-Pro?no
- [IMPORTANT] Related original log or error message is required if you're experiencing an issue.
====
Getting email reports from newly created iRedmail servers like this one:

Mail Delivery System <MAILER-DAEMON@mlr2.powersend.net>
Postmaster Copy: Undelivered Mail
To: postmaster@mlr2.powersend.net
<netdata@mlr2.powersend.net>: user unknown
and
<root@mlr2.powersend.net>: user unknown

Getting similar messages from another server mlr1.powersend.net.

AND other warning messages like:
netdata@srv1.powersend.net
srv1 needs attention - 1m ipv4 udp receive buffer errors - ipv4.udperrors
1m ipv4 udp receive buffer errors = 3.67 errors
number of UDP receive buffer errors during the last minute ALARM

These servers were created with the free version of iRedmail.
I don't remember seeing errors like this when creating a server with the Pro version.
But I will double check.
I am presuming the Pro version we paid for was for only 1 server....so I used the free version on these other 2.

Can you help me with this?
I'm still learning the Linux email technologies.
Thanks