monit hostname is always localhost

monit hostname is always localhost

I've got monit running on a server to alert by email with the following
config
set mailserver 127.0.0.1
using hostname "vps1.sender.com"
set mail-format { from: monit@sender.com }
set alert webmaster@receiver.com
but the mails are still coming through with a hostname of localhost
Aug 7 08:40:55 mx1 postfix/smtpd[31060]: NOQUEUE: reject: RCPT from
vps1.sender.com[xx.xx.xx.xx]: 504 5.5.2 <localhost>: Helo command
rejected: need fully-qualified hostname; from=<monit@sender.com>
to=<webmaster@receiver.com> proto=ESMTP helo=<localhost>
It's the only config file I have.
What am I missing?