Thank you Typhoon for pointing me to the right direction.
Since we are running Exchange 2016, and its Receive Connector already configured to allow 500 simultaneously, I went into WHD's Email options, and changed the Scheduled E-Mail Delivery Threshold from 5 to 7. Will monitor for a few days and see whether that fixes it.
Thank You Again,
Wishing you a good day, I am receiving same error but the difference is that it is not maintaining one number, some time email goes to 5 person some time 3 like that randomly changes, I want to know if this will be exchange problem then it should always maintain one number for example each time it should send 5 email and discard more than 5 if the limit will be 5.
but in my case it is randomly changes some time email goes to all recipients, we have not much emails in bulk emails may be the highest number will be less than 20.
one last thing I want to mention that I am facing this problem one in WHD if a try to send one email to multiple recipients form outlook I do not get any error like that,
Kindly explain that what will be issue for that.
Thanks in advance