If the To or CC header of an email contains entries that lack an '@', an oops like OOPS-1803C4636 will result.
Such emails are unlikely to be generated by users, because normal mail programs usually determine the SMTP TO from the user-entered To, CC and BCC values, and copy them into the To and Cc headers. The email that produced this oops was spam.
This bug could be fixed by making processing more tolerant of invalid email addresses, or by using X-Original-To instead of To+CC. X-Original-To will always be a valid address, since it's inserted by the mail delivery system.