We are currently running exchange server 2007 and are in the process of moving towards Office 365 for our email needs by the year end.  

My question is specifically towards exchange and/or Outlook, but also would be applicable within the office 365 environment.  Is there any way to limit the number of email recipients in the to field specifically?  IE, if someone is sending an email to more than X people, then it will alert the user that it cannot be sent, without the same limitation applying to the BCC field?

The use case would be to prevent the accidental sharing of email addressed during mass mailings.

You can always restrict that value by running the below command 

Set-TransportConfig -MaxRecipientEnvelopeLimit 1000
When the sender adds more than 1000 recipient he will be getting a notification that he cannot send since he exceed the recipient limit.

I am aware of this, but as this is at the transport layer, I don't think that it would prevent the email from only going out if those recipients were in the TO field.  In this case, the limit is applied across the board, whereas my intention is to prevent the user form sending them only if the emails will be disclosed to the other recipients.  BCC for 1000 recipients would be allowed, whereas TO for 1000 recipients should fail.  

I am not sure if this exists, however my examples above would be well higher than the number of recipients that we would be looking to restrict.

Hi,

Based on my knowledge, Exchange has no built-in function to restrict the recipients only in the TO field. The workaround is to use the cmdlet Sathish provided to restrict the recipients including TO, CC and BCC field. Your understanding will be appreciated.

For your reference:

Set-TransportConfig

https://technet.microsoft.com/en-us/library/bb691276%28v=exchg.80%29.aspx?f=255&MSPPError=-2147217396

Hope this can be helpful to you.

Best regards,