"Undelivered Mail Returned to Sender" for admin@domain.tld on each cron run

My YunoHost server

Hardware: VPS bought online
YunoHost version: 11.2.3 (stable)
I have access to my server: Through SSH
Are you in a special context or did you perform some particular tweaking on your YunoHost instance? : no

Description of my issue

Every time a cronjob runs on my yunohost server, an email with the cronjob output is sent to admin@mydomain.tld. This email in turn sends an “Undelivered Mail Returned to Sender” message to root@mydomain.tld, making the root@ inbox feel quite spammy.
How do I make it stop?

I’ve grepped the entire /etc directory for admin@ and didn’t find anything useful. I’d be thankful for any tips on where to find the place that configures the CRON email address

I have the same issue. Since the admin user was removed during a Yunohost upgrade, I receive a mail “Undelivered Mail Returned to Sender” each time a mail is sent by Yunohost.

The error is:

<admin@mydomain.tld> (expanded from ): user unknown

I didn’t find were is admin@mydomain.tld in my server.

sudo grep -r admin /etc/cron*

For example, the undelivered mail is:

From: Cron Daemon root@ynh.mydomain.tld
To: root@ynh.mydomain.tld
Subject: Cron root@ynh test -x /usr/sbin/anacron ( cd / && run-parts --report /etc/cron.daily )

It doesn’t contain the admin email.

It seems to be an issue in the email configuration.

I just realized I have a admin user:

sudo yunohost user info admin
Warning: E-mail turned off for user admin
fullname: Admin
loginShell: /bin/bash
mail: admin_legacy
  limit: No quota
  use: ?
username: admin