some mails of mass newsletters delayed 24 hours

[baijianpeng]

I use the mail server of VestaCP to send out newsletters to registered members on my website. In the Mail Server settings of my newsletter script ( AcyMailing component for Joomla CMS), I had choose "SendMail" as the sending method, and Acymailing had automatically deteced the path of SendMail is:

/usr/sbin/sendmail

Then I started to send out about 20k emails, by a frequency of 100 emails then pause 5 seconds, then another 100 emails then pause, and so on.

However, after sending process finished, I got some "bounce" emails, many of them have following subject:

Warning: message 1agnyK-0005IH-8t delayed 24 hours

For example, one of them has following email body:

This message was created automatically by mail delivery software.
A message that you sent has not yet been delivered to one or more of its recipients after more than 24 hours on the queue on iK239i3uk4be.

The message identifier is: 1agnyK-0005IH-8t
The date of the message is: Fri, 18 Mar 2016 06:33:16 +0000
The subject of the message is: =?utf-8?B?Wm9vIOWVhuS4muaJqeWxleWboui0remihOWUruaIkOWKn++8jOS7iuaXpQ==?=
=?utf-8?B?6LW35q2j5byP5byA5Zui?=

The address to which the message has not yet been delivered is:

yang***@domintec.com

No action is required on your part. Delivery attempts will continue for some time, and this warning may be repeated at intervals if the message remains undelivered. Eventually the mail delivery software will give up, and when that happens, the message will be returned to you.

I am confused. Why those emails (more than 300) were delayed? Because the sending task had put my server overload? Does this means I should set a lighter sending load, such as less than 500 emails per minute?

If this is not related to server load, then how to solve this? It seems that most of those delayed emails got returned (not sent out) to me in the end.

Thank you.

[skurudo]

May be something else in logs before message been frozen?
There must be some reason.