Issues with Emails from my Nextcloud instance

[pflavio]

Steps To Reproduce

1. Set-up Email under "basic settings" with address "[email protected]"
2. Send test email -> no issues, received from "Nextcloud [email protected]" to "[email protected]"
3. Send passwort reset emails to users -> no issues, received from "Nextcloud [email protected]" to "[email protected]"
4. Every morning at 10:27 receive an email from "Mail Delivery System MAILER-DAEMON@nextcloud" to "root@nextcloud" with subject "Undelivered Mail Returned to Sender" and content

This is the mail system at host nextcloud.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<[[email protected]](mailto:[email protected])> (expanded from <root>): host
    [mail.protonmail.ch](http://mail.protonmail.ch/)[185.205.70.128] said: 504 5.5.2 <root@nextcloud>: Sender
    address rejected: need fully-qualified address (in reply to RCPT TO
    command)

plus some logs. The addresses used in these automatic emails don't have a tld, but I can't find where to set (and possibly disable) those. The Email credentials I entered personally in the admin panel work as expected.

Expected Result

Either receive no automated emails or receive them using the credentials provided.

Actual Result

Receive aan undeliverable notice every day.

Screenshots, Videos, or Pastebins

No response

Additional Context

No response

Build Version

30.0.6

Environment

By using the scripts

Environment Details

No response