Bounce Back - Only for some on an email to multiple people

We are using Server 2012 with Exchange 2013 installed, my boss has an excel list with email addresses that he copies and paste from.  All the email addressed on the list are confirmed and work on an individual basis.

He is using Outlook 2013, when he copies multiple over to the To column not even the BCC column but the To section in Outlook he gets the below bounce back for some.  It is never the same people that get the bounce back either, it is random with the errors below.  So say out of 50 he might get the bounce back 10 times, everytime, but it is always 10 random people.  

I read that 500 is the default setting for max recipients in both outlook 2013 and exchange 2013 but can't find where to check that anywhere.   Again all addresses are confirmed correct.

Here is a sample, he gets an email with this same bounce back for however many don't through that time. rejected your message to the following email addresses: ( gave this error:
User Unknown

The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.

Diagnostic information for administrators:

Generating server: FT-EXCHANGE.fosterthomas.local
Remote Server returned '550 5.1.1 User Unknown'

I x'd out the name and domain because they are a big client but he can email them indiviudally just fine but when he copies over and the email is confirmed to have copied properly.  He gets that bounce back.

Any Ideas?
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Your max recipients is listed under Organization Configuration > Hub Transport > Global Settings tab and then the Transport Settings (right-click Properties).  It's listed on the General tab.  I do believe "500" is checked by default.

Have you checked the sender's individual max recipients under the user's mailbox?  You can set the max recipients under the Mail Flow Settings tab and then selecting Delivery Options > Properties.
FosterThomasAuthor Commented:
It is different now in 2013 that is why I am so confused, there are no tabs any more it is through the ECP console and nothing is where I think it should be.

Instead of going on the server I go to for all these edits and they are in a hotmail/owa type set up.

I can't find the Hub Transport or global settings in either ECP or the users mailbox
In the left column in ECP there isn't an option for "mail flow" for each user mailbox and "servers" to manage the organization?
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FosterThomasAuthor Commented:
Yes I went to both Organization and Mail Flow and I can't find this setting under those options.  

For Mail Flow I get sub options of Rules Delivery Reports, Accepted Domains, Email Address Policy, Receive connectors, send connectors

Under organization i have Sharing, Apps, Address List

I've gone through every setting in all those sub cats and can't find it in there, unless i am just overlooking it.
Mail flow > Receive connectors > More options/More Options Icon > Organization transport settings > Limits tab > Maximum number of recipients.

It appears that the individual user recipient setting must be done via the Exchange console.  I'm guessing there is no limit set on the individual mailbox because the default is unlimited.

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FosterThomasAuthor Commented:
that is set at 20240??
Max recipients for the organization is set to 20240?  I have something close to that for max message size, not recipients.
FosterThomasAuthor Commented:
yes Max Recipients.

So I am not sure why else the bounce backs would be happening?
I'm going to start guessing here - but maybe he's violating another policy, such as an anti-SPAM (too many recipients) policy?  Has he tried mailing out in two blocks of say 25 recipients each?
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