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Old 08-03-2010, 08:45 AM   #1
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Sendmail - fight against the spammers

I am new to Slackware and sendmail.

1) I want to make sendmail more resist from spammers. Can You suggest some lines in (My server is Can I do, that my server recieves mails, but users cannot send mails with SMTP through my server?

2) Many mailservers returned mails, because I have no reverse DNS. What I must to do?

Last edited by XavierP; 08-03-2010 at 11:28 AM. Reason: Moved to Security
Old 08-04-2010, 05:28 PM   #2
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Elmucis, Welcome to

In general terms you will want to make sure that your email (smtp) server is NOT what is called an open relay. An open relay will accept mail from (any) non-authenticated domain and forward it to the next hop. As you know, spammers typically look for open relays to work through. You will want to configure your server to use authentication. It appears that sendmail uses two methods for this: SMTP AUTH and Cyrus SASL. I am not familiar enough with sendmail (I use postfix) to tell you specifically how to do this, but you should consult the documentation and the how - to documents.

With respect to your other comment, it sounds like you are trying to run an email server via a dynamic IP address. As a general rule, mail from dynamic IP address blocks will be rejected as > 99.999% of it is spam from a hiacked machine. If you are running your server from a PUBLICLY static IP address (not static on your LAN), then you may need to add what is called an SPF record. An SPF record is a DNS function that allows a receiving MTA to do a reverse lookup and verify that your system is declared as being a mail sending agent. Obviously a system that has been hijacked by malware and is sending spam will not have an SPF record.



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