When using zabbix_sender -c /your/zabbix_agentd.conf, it is needed to declare explicitly Hostname=your-hostname.

It does not behave like the agentd bin that queries the OS for hostname.

If i declare Hostname=my-hostname, it works ok, as below:

[root@my-hostname ~]# zabbix_sender -c /etc/zabbix_agentd.conf -k log_nagios_restart -T -o 15
info from server: "Processed 1 Failed 0 Total 1 Seconds spent 0.000055"
sent: 1; skipped: 0; total: 1
[root@my-hostname ~]#