@roberto & Co. 
Spent some time, over these errors. 
It took me a long time because I am not a programmer.
It seems that the cause of these messages is unsent messages.
The DMA is trying to send them, but due to previous errors and fails.
===
What I did.
- created files
/var/mail/root
/var/mail/nobody
After this action, the above errors stopped appearing.
- added to the file
/var/ipfire/reports/send_mail.sh
# Get sender email
SENDER=$(grep -E '^SENDER=' "$MAIL_CONF" | head -n1 | cut -d'=' -f2- | tr -d ' ')
if [[ -z "$SENDER" ]]; then
echo "Error: No se encontró SENDER en $MAIL_CONF" >&2
exit 1
fi
the following line
} | $SENDMAIL -t
I changed to
} | $SENDMAIL -f $SENDER -t
===
After these changes
1 in the file `/var/ipfire/dma/ I removed the line “MASQUERADE”
2 I deleted the files
/var/mail/root
/var/mail/nobody
3 I did a test of sending reports
The test looks successful. 
===
Could someone with more proramist experience check the correctness of the changes made?
Best Regards