Have you checked the parent directories and the ownership of whatever script is calling it and what are their permissions?
I can't reproduce your case with a normally installed postfix, so maybe something you installed changed some parent directory permissions:
-bash-3.2$ uname -a
Linux asl-modsec-test.gotroot.com 126.96.36.199-6.art.x86_64 #1 SMP Thu Jul 14 14:14:48 EDT 2011 x86_64 x86_64 x86_64 GNU/Linux
uid=48(apache) gid=48(apache) groups=48(apache),1005(untrusted)
-bash-3.2$ ls -al /usr/sbin/sendmail*
lrwxrwxrwx 1 root root 21 Sep 19 2010 /usr/sbin/sendmail -> /etc/alternatives/mta
-rwxr-xr-x 1 root root 201784 May 31 12:29 /usr/sbin/sendmail.postfix
-rwxr-sr-x 1 root smmsp 775064 Aug 11 13:24 /usr/sbin/sendmail.sendmail-bash-3.2$ echo test | /usr/sbin/sendmail.postfix root@localhost
One other thought, are you sure its phpmailer thats doing this? That message says it was a shell that did it, which implies interactive action is occurring. Is some other script calling sendmail.postfix? Or maybe a malicious user?
but can you advise as to how the upgrade would have caused this?
Do you mean a Plesk upgrade or something else? If you mean Plesk, you may want to ask Parallels what changed. you may also want to ask the phpmailer folks how their software works, because it looks very odd to me. I'm not sure this is the software being called, look carefully at the message:
kernel: grsec: From 188.8.131.52: denied untrusted exec of /usr/sbin/sendmail.postfix by /bin/bash[sh:18055] uid/euid:48/48 gid/egid:48/48, parent /usr/sbin/httpd[httpd:17879] uid/euid:48/48 gid/egid:48/48
That means /usr/sbin/sendmail.postfix was called by bash, a shell. Thats pretty strange for apache to call something via the shell. Are you sure your application is using an interactive shell? If it is, how does it do this? Thats a very odd way to call things from apache, normally I'd expect to see PHP, PERL or something else calling sendmail.postfix.
I'd try phpmailer myself, but I'm not familiar with phpmailer and it looks like its not a stand alone app but rather something you include in another app?