Sending mail with KMail using DBus

Hello to everyone,
I'm trying to write a script which uses KMail to send automatically generated 
mails to several people using DBus. I used QDBusViewer to find out which 
methods KMail provides and I saw a sendMessage method which takes the various 
fields of the mail (From, To, ...) as parameters and sends the mail. I tried 
using it from QDBusViewer and it worked correctly. However, when I try to call 
it from the command line using qdbus, it doesn't send the mail.

The command I use is:
qdbus org.kde.kmail2 /MailTransportService  
org.kde.mailtransport.service.sendMessage [email protected] [email protected] "" "" 
test Test ""

This call returns true but no mail is actually sent. 

I tried monitoring the DBus calls using dbus-monitor and, from what I can see, 
the calls made using QDBusViewer and qdbus are identical.

This is what I get using qdbus:
method call time=1586454311.104053 sender=:1.392 -> destination=org.kde.kmail2 
serial=5 path=/MailTransportService; interface=org.kde.mailtransport.service; 
member=sendMessage
   string "[email protected]"
   string "[email protected]"
   string ""
   string ""
   string "test"
   string "Test"
   array [
   ]

And this is what I get when using QDBusViewer:
method call time=1586454543.815489 sender=:1.368 -> destination=org.kde.kmail2 
serial=182 path=/MailTransportService; 
interface=org.kde.mailtransport.service; member=sendMessage
   string "[email protected]"
   string "[email protected]"
   string ""
   string ""
   string "test"
   string "Test"
   array [
   ]

The only difference I can spot between the two outputs is the "serial" value 
but, from what I understand, that is not something you can decide yourself.

Does anyone have any idea on what I'm doing wrong?

Thanks in advance

Stefano