rsyslog ommail uses wrong helo command when sending email

I have set up ommail module in rsyslog for sending emergency emails:

$template mailSubject,"EMERGENCY"
$template mailBody," %$NOW% %timereported:8:15%  %programname%: %msg%\n"

$ActionMailSMTPServer               1.2.3.4
$ActionMailFrom                     root@myserver.com
$ActionMailTo                       user@foo.com
$ActionMailSubject                  mailSubject
$ActionExecOnlyOnceEveryInterval    60
*.emerg                             :ommail:;mailBody

When I test it with logger, it attempts to deliver the email to the mailserver 1.2.3.4, but is rejected because it uses hostname as helo command, instead of fqdn.

In syslog on the local machine, I see this message:

rsyslogd:  action 'action-10-ommail' suspended (module 'ommail'), retry 0. There should be messages before this one giving the reason for suspension. 

And on the mailserevr 1.2.3.4 the postfix maillog has this error:

Helo command rejected: need fully-qualified hostname; proto=SMTP helo=<host>

so I see that rsyslog is using host as helo command. But my fqdn is host.myserver.com.

This seems to be configured correctly on my side:

$ hostname 
host
$ hostname -f
host.myserver.com

why is rsyslog not using fqdn for helo command ?

How can I tell it to use fqdn ?