Mastodon / Glitch-soc won't send mails

My YunoHost server

Hardware: Fujitsu Siemens Esprimo Q920 Intel Core i5-4590T 64bit CPU @ 4 × 2,8 GHz, 16GB RAM, 8TB SSD
YunoHost version: (stable)
I have access to my server : Through SSH | through the webadmin | direct access via keyboard / screen | …
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : yes
If yes, please explain: The yunohost-installation itself is unaltered. But I added task-xfce-desktop, which is not started by default. I only start it, if needed (which is rarely the case) and end it immediately afterwards again.
If your request is related to an app, specify its name and version: glitchsoc 2024.03.20~ynh1

Description of my issue

Mails can’t be sent from Glitch-soc to e.g. new users. I checked the variables in .env.production and as far as I can tell, they are wrong.

To be honest, I’m hesitant to chattr -i and manipulate the configuration file in a production environment, so I’d like to assure myself by asking here, whether that’s really the supposed way of making changes?

Sending mails from my firefish installation wouldn’t work after yunohost installer finished either. Fortunately I could change the SMTP variables comfortably in the admin backend there. Sadly this isn’t an option with Mastodon / Glitch-soc.

This is the relevant part of the .env.production:

# Sending mail
# ------------
SMTP_DELIVERY_METHOD=sendmail # delivery method can also be smtp

Is there another way to enter the correct values?

Thanks for your help!