If you can look at the Internet Headers for the mail, you may be able to trace backwards using the IP addresses.
I was subject to several abusive emails from a former colleague, and traced him to his new employer, and provided them with the internal IP address of the PC where the webmail was sent from. They acted pretty swiftly to stop his actions using their IT equipment.
If you are using Outlook, open one of the mails and open it’s properties (in 2010 it is under the File Tab). The Internet Headers are located there.
If the IP address trail leads to a work PC, then report immediately to your IT department/Admin.
Use WhoIs to trace the IP addresses.