[ic] Suddenly some order emails don't get sent ??

[DB]

IC 4.8.6 with a modified foundation. I've had hundreds of orders without 
a problem. Now maybe 1 in 3 order email don't get sent and I must get 
the credit card info from the error log. I don't see anything in the 
archives about this. I've made no changes in my Interchange 
configuration recently. Some order email still get through ok. The 
actual error message says:

/cgi-bin/store/process.html Unable to send mail using /usr/lib/sendmail

The server runs Redhat 7.1 and I did make one change around the time 
this behavior started. I applied the Redhat security update to glibc 
shown here:
https://rhn.redhat.com/errata/RHSA-2002-197.html

Could this have caused my problem? Should I undo it? Any other ideas on 
how to solve this annoying problem? All info appreciated.

DB