First, it appears that the -z (server) option is now a required command line option - I was using zabbix_sender 1.4 with an input file, where each line specified the server option and therefore was not previously specifying server on the command line. This no longer works, and this change is not clearly documented.

Secondly, it appears that the expected format for input files has changed! Although the help message and Zabbix 1.6 documentation still state that each line should be:

<zabbix_server> <hostname> <port> <key> <value>

it appears that the actual 1.6 expected format is:

<hostname> <key> <value>

It took me a while to figure this out, I finally did by looking in the zabbix_sender source code.

The fact that this is not documented properly is a bug and should be fixed!

(this is not to mention the fact that these changes will probably break a lot of scripts that rely on zabbix_sender in the case of upgrade, but too late for that!)