[Koha] Koha and Nullmailer: can they work together?

[Odyssey Laertes]

I have Koha 3.18 installed on my Ubuntu notebook for testing purposes, and
right now I am trying to make Koha's emailing work. Since this is not meant
to be a production environment I have not need (and prefer to avoid) to
install a full-blown mail server, like Postfix. I have Nullmailer installed
and it relays the messages from cron jobs through my gmail account. It
works fine. I was hoping that Koha would be able to do the same but it
looks like it does not work with Nullmailer. I get error messages Access
Denied and my best guess it is happening because it tries to contact
localhost on port 25 and Nullmailer does not listen on port 25. My Drupal
sites that are sitting on the same notebook, for comparison, do not have
problems with sending e-mails through Nullmailer.

Does anybody know if it is possible to make Koha work with Nullmailer?

Nataliya