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I am trying to set up an email server that receives email from a mx server using sendmail software on Fedora 30. When the mx forwards email to the email server, the logs show a Relaying denied - Proper authentication required error and also indicate that sendmail is attempting to authenticate the sender instead of the receiver. I have never seen it do this. Why would sendmail try to authenticate the sender?

Furthermore, why should sendmail be trying to authenticate incoming emails from an mx server? It should just be filing them in the appropriate inbox.

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    You have not provided your config file (an Sendmail - yuk. Use Postfix!). The issue is likely that your mail server does not recognise the IP addresses that are connecting to it - you need to add these (If it were postfix you would just add them to "mynetworks". Postfix is trying to authenticate the sender so the server is not used as an open relay and used to send spam.
    – davidgo
    Feb 21, 2020 at 4:48
  • Good advice. I added this to access file and that eliminated the error: Connect:<MX ip> RELAY ... now I have a new problem ... sendmail SYSERR config error: mail loops back to me (MX problem?) ... I wish I could use postfix ... but I don't think mimedefa-filter is compatible with it Feb 21, 2020 at 5:40
  • There is very little you can do in sendmail you cant do in postfix. Its a drop in replacement. Also - serverfault.com/questions/542679/…
    – davidgo
    Feb 22, 2020 at 0:44

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davidgo provided the generic answer in a comment so I am re-posting the more specific answer here.

I added this to the /etc/access file and that eliminated the error:

Connect: RELAY

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  • Have it made your server an open relay? [It *is BAD]
    – AnFi
    Feb 26, 2020 at 0:20
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Fixing list of local email domains (delivered to local mailboxes)

Fix sendmail's list of "local email domains" (domains delivered to local mailboxes).
Sendmail accepts such recipients with no need to authenticate.

To check current list of local email domains as root execute: echo '$=w' | sendmail -bt

To add domains to list of local email domain append them to text file /etc/mail/local-host-names. Restart or HUPing sendmail daemon is required to make it see the changes.

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