Received messages bouncing with 421 4.3.0 because of os error 13

KumoMTA Troubleshooting
1

I’ve successfully configured my Kumo instance to be able to send an email.

However, something is clearly wrong on the receiving side.

When monitoring for an inbound message with

[root@srv1081723 ~]# journalctl -f -n 50 -u kumomta.service

I see the following message

Oct 24 17:19:18 xxx.cloud kumod[6697]: 2025-10-24T17:19:18.216271Z ERROR smtpsrv-2 run{socket=PollEvented { io: Some(TcpStream { addr: xx.xx.xx.xx:25, peer: xx.xx.xx.xx:57371, fd: 35 }) }}: kumod::smtp_server: Error in SmtpServerSession: Permission denied (os error 13)

os error 13 usually indicates a resource that the process doesn’t have access to, but I can’t figure out what resource it’s failing to access, even after turning logging up to TRACE.

I’m also to send from and to myself- that is, injecting SMTP via telnet with:

ehlo moto
mail from:me@mydomain.com
rcpt to:me@mydomain.com
DATA
from:me@mydomain.com
to:me@mydomain.com
message-id:dceb7af5-5432-45ee-a722-6132b1088fd5
subject: Test Email

Hey, this is my first email!

Yields a message being delivered to /var/maildirs/<mydomain.com>/me/new

4

It is most likely a TLS permissions error. You have not shared your configs but that is pretty typical