koji-gc --action=trash in fedora is currently failing with
Traceback (most recent call last):
File "/usr/sbin/koji-gc", line 908, in
rv = main(args)
File "/usr/sbin/koji-gc", line 417, in main
globals()'handle_' + x
File "/usr/sbin/koji-gc", line 525, in handle_trash
send_warning_notice(owner_name, [x[1] for x in builds])
File "/usr/sbin/koji-gc", line 410, in send_warning_notice
s.sendmail(msg['From'], msg['To'], msg.as_string())
File "/usr/lib64/python2.7/smtplib.py", line 746, in sendmail
raise SMTPRecipientsRefused(senderrs)
smtplib.SMTPRecipientsRefused: {'dionysos@fedoraproject.org': (550, '5.1.1 dionysos@fedoraproject.org: Recipient address rejected: User unknown in local recipient table')}

the users account is no longer active and the email is no longer valid. rather than givinga traceback and failing koji-gc should log that sending failed and move on.