Spamc timeout for emails sent to 2 accounts on same domain

Undelivered Mail is returned to Sender, with error:
Command time limit exceeded: "/usr/bin/spamc"….

Background:
CWP7pro, Spamassassin 3.4.6
spamc is called from postfix master.cf, using

spamassassin      unix -      n      n      -      -      pipe
  user=vmail argv=/usr/bin/spamc -u ${recipient} -f -e
  /usr/sbin/sendmail -oi -f ${sender} ${recipient}
Source: http://wiki.centos-webpanel.com/how-to-configure-spamassassin

Email contains an embedded image.  Email size is 1,605,413 bytes
Problematic Email is being sent to 2 recipients on the same domain fails

Notes:
Sending email to 1 recipient is OK
Sending email to 2 recipients on different domains is OK

Tried doubling spamc timeout, eg in postfix master.cf
  user=vmail argv=/usr/bin/spamc -u ${recipient} -t 1200 -f -e
then restarted postfix & spamassassin

SpamAssassin is running fine in all other regards. 
Has anyone seen anything like this?  Can someone guide me into why this might be happening and how I can fix it?
Thanks in advance.