[Snappymail] Can't send email

[lnoferin]

My YunoHost server

Hardware: Raspberry Pi at home
YunoHost version: 11.2.9.1
I have access to my server : Through SSH | through the webadmin
Are you in a special context or did you perform some particular tweaking on your YunoHost instance ? : no
If your request is related to an app, specify its name and version: snappymail 2.31.0~ynh1

Description of my issue

I can’t send email because of an error like (simply) Can’t send email displayed on the browser.

I don’t know where to look for some debug. When I try to send email I can read this on journalctl:

Jan 08 20:29:33 server.local postfix/smtpd[14560]: connect from localhost[::1]
Jan 08 20:29:33 server.local postfix/smtpd[14560]: NOQUEUE: reject: RCPT from localhost[::1]: 553 5.7.1 <utente1@server.local>: Sender address rejected: not logged in; from=<utente1@server.local> to=<utente2@receiver.local> proto=ESMTP helo=<receiver.local>
Jan 08 20:29:33 server.local postfix/smtpd[14560]: disconnect from localhost[::1] ehlo=1 mail=1 rcpt=0/1 quit=1 commands=3/4

[Magellan]

Maybe you need to unlock the SMTP authentication first. Log in to Snappymail admin backend (https://my.domain.tld//snappymail/app/?admin). Check under “Domain” to see if it is activated.

[lnoferin]

Thanks.
It was written also in the configuration panel but I did not see it before!