When updating to 10.1.5, the installer replaces sendmail with (presumably) an updated version as part of the install. If you have switched to postfix, you most likely have a compatability app at /usr/sbin/sendmail. However, the Apple update removes this and replaces it with an actual copy of sendmail.

You will probably want to replace this actual copy of sendmail with the compatability app or strange things might happen (or more likely nothing will happen). The script can be found in in the bin subdirectory of the postfix source directory if you built it from source. If you didn't, then you may need to reinstall the file some other way.

A point to note is that if you copy the file from the bin subdirectory of your original source directory, then you should perform a chmod -r sendmail command on it or you will get complaints in your mail log about security risks.

Well, I am still using Sendmail, and haven't changed from it. But after updated to 10.1.5 my PhP mail() routines fail. Did something else change in Sendmail that makes it not work??

I've found a workaround, using a class.smtp.php file I found that allows me to use Communigate instead of Sendmail to email from PhP. Which I honestly like better---but it still stinks that the update nullified my PhP scripts....