You are receiving the following error when attempting to send mail in Apple Mail
Apple Mail is attempting to send your email through your Exchange (@pthsd.k12.nj.us). As of 12/28/2015 this email address not longer has the ability to send email messages.
Please Note: You will need to recreate any Messages that have not been sent and are sitting in your PTHSD.K12.NJ.US Outbox as unsent.
Please complete the following steps to resolve this issue:
1. Click on the Apple Icon in the upper right hand corner
2. Locate and click on System Preferences
3. Locate and click on the Internet Accounts Icon
4. Locate and click on PTHSD.K12.NJ.US to highlight it then click the minus icon
5. On the pop up box that come up (see picture below) click OK.
Once these steps are completed the your Exchange Server Account will be removed from the Mail Application and the PTHSD.Net account will become the only email address used in the program, resolving your issue