Unlimited mail send retries with non-existent / deleted mail address

Steps to reproduce

1. Add an account with a non-existent mail address (here the mail address comes from an LDAP Source, but may also accour with deleted mail adresses on normal accounts; mail address is in our domain, see mailserver note below)
2. Create some activity e.g. share something with this account

Here I have to add, nextcloud uses directly our domain mailserver via smtp without a local MTA. This mailserver will verify RCPT and return an error on non-existent RCPT mail addresses inside our domain directly (55x on RCPT) .

Expected behaviour

In the cronjob, after failing sending the mail, retry a limited number of times, e.g. 3. If it's still failing again, log a warning and do not try again sending mails / remove entries from mq.

Actual behaviour

Cronjob tries to send a new mail on every call (15 Minutes), filling up the mail server log with error messages.
Notification is never removed from activity_mq.

Server configuration

Operating system: Debian 10
Web server: Apache / php_fpm
Database: MariaDB
PHP version: 7.3.22
Nextcloud version: (see Nextcloud admin page) 19.0.6
Where did you install Nextcloud from: Upgrade
List of activated apps: Activity 2.12.1, LDAP 1.9.1

Are you using an external user-backend, if yes which one: LDAP

Client configuration

Browser: Firefox
Operating system: Win 10

Logs

no error messages in Apache/Browser/Nextcloud logs.
But mailserver log fills up with many tries to send to an non-existent mailadress.

Start at the Activity cron job's failed-send handling and the activity_mq queue described in the report. Reproduce a send to a non-existent address and trace how repeated failures are queued. Done means failures are retried only a limited number of times, then logged and removed or suppressed from activity_mq.