I have a very urgent issue, a few days ago sent emails come back with the following message:
This is the mail system at host NethServer.tttttttt.it.
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
firstname.lastname@example.org: host smtp.yyyyyy.eu[18.104.22.168] said: 552 5.2.0
QBCH1s01Q4Sljan01BCJEk spam detected (in reply to end of DATA command)
Until now I used smarthost inserting the parameters of my ISP so as to avoid the emails identified as spam.