Why can't the nagios user send mails using "sendmail"?

Edit: after extensive manual interrogation of your SMTP relay, it looks like the problem may be that the relay specifically blocks nagios@ as a sender:

[izx.I5] ? telnet smtp10.netcore.co.in 25
220 ESMTP
EHLO ********.com
...
MAIL FROM:nagios@********.com
250 2.1.0 Ok
RCPT TO:karthick@********.com
450 4.7.1 : Sender address rejected: Please checked sender domain
quit
221 2.0.0 Bye
Connection closed by foreign host

Does the nagios@****.com mailbox exist? Can you try simply rewriting nagios to e.g. nagios1?

To do this, in /etc/sendmail.mc add:

FEATURE(`genericstable')dnl
GENERICS_DOMAIN_FILE(`/etc/mail/generics-domains')dnl

Create file /etc/mail/genericstable and add the line:

nagios nagiosk

and /etc/mail/generics-domains and add lines:

localhost
localhost.localdomain

Then run sudo sendmailconfig and try again.

The issue appears to be your SMTP relay at netcore.co.in. Hosting/ISP SMTP relays are never "open", and usually set to only relay mails from one of "their" domains and from one of "their" IPs. Mails from root@localhost are probably getting through only because some mail server daemons recognize root and make an exception for it, sometimes even replacing the @localhost part with whatever the reverse DNS (or IP) of the sender is (for example, my messages once went through in the form of [email protected]). No such courtesy is afforded to the "nagios@localhost" user.

Two solutions:

The easier one may be to simply set (or fake) your sendmail FQDN to whatever is recognized/authorized by Netcore.

To do this, open /etc/mail/sendmail.mc and add these lines at the end before any MAILER lines, modifying appropriately:

MASQUERADE_AS('karthick.com')
FEATURE(`masquerade_envelope')

Then run sudo sendmailconfig, answering yes to everything and make sure sendmail is restarted. See here for some more basic info on Sendmail masquerading for relaying.

Alternatively, for the sake of completeness, to keep using nagios@localhost, you can do one of two things -- either disable relaying in sendmail and use it directly (if you have a static IP/PTR record or RDNS), OR you can set up sendmail to perform SMTP authentication with the Netcore relay.

These Netcore articles may help; you can try asking them too if possible:

• Why does a sender address get rejected?
• SMTP authentication for Netcore

First of all: Checking the web arround, and by looking at your logs, it doesn't seem to be an issue with the user nagios, rather, it seems to be an issue with sendmail's configuration.

To be more accurate, a user in this forum suggests he gets a similar 450 error code (he was using postfix, but I assume that there is some kind of protocol) which, as in your situation too, denotes a domain not found issue. To quote another user from that same forum:

This is usually due to a misconfiguration of your e-mail client if the sender address is for a non-existent domain. It could also be, if yours is a new domain, that the receipent's mail server has not caught up with DNS propagation yet.

While you are using localhost as a sender domain, I still believe that there is more that can be taken care of.

As a matter of fact, another user from another forum I stumbled upon, seems to have a similar problem to the user in the first forum I linked to, and he too gets the same error code: 450. In that same forum another user is quoted suggesting that it is a DNS issue too.

Important: I am aware that you are using localhost and not a particular domain. However, to me it seems like it must be an misconfiguration issue of the sendmail program, rather than user nagios not having rights or anything.

I have also found some other sources that I am researching currently. Will continue to update this answer as I get more and more information on the matter.

[EDIT #1]: Another user in another forum is quoted saying:

It appears you are sending from [email protected], when the receiving server does a reverse lookup on this address it will fail as it is not valid and hence the mail will be rejected correctly with the error you have given. Ensure you have a valid server name/domain for your server and that it can be looked up in dns both forward and reverse.

However the user asking the question there doesn't seem to face a 450 error code. However he gets a similar answer to what I have seen so far on the interwebz.

[EDIT #2]: The more I read about your problem, the more I get it: It seems like nagios@localhost that you have selected to use as the domain that sends the emails doesn't resolve to a valid Ip address, and that's what's causing the smtp10.netcore.co.in server to respond with a 450 domain not found response, for it believes that your mail is spam:

Yep,"reject_unknown_sender_domain" can cause a lot of false positives, but it does cut down on the spam.

The most probable fix is to make sure that you use a domain that is resolvable.

In my nagios configuration the command for sending mail ist

/usr/bin/printf "%b" "***** Nagios *****\n\nNotification Type: $NOTIFICATIONTYPE$\n\nService: $SERVICEDESC$\nHost: $HOSTALIAS$\nAddress: $HOSTADDRESS$\nState: $SERVICESTATE$\n\nDate/Time: $LONGDATETIME$\n\nAdditional Info:\n\n$SERVICEOUTPUT$\n" | /usr/bin/mail -s "** $NOTIFICATIONTYPE$ Service Alert: $HOSTALIAS$/$SERVICEDESC$ is $SERVICESTATE$ **" $CONTACTEMAIL$

Try out if you can send mail as user nagios from the command line:

su nagios
echo "test sendmail" | /usr/bin/mail -s "test" [email protected]

This may either fail because of a bad path, access rights or the mailserver not accepting the destination address. Please keep us posted on your test results.

Sendmail is kinda ancient. While some people think of it as the best mail server ever written, personally I do not enjoy it the least bit.

My combination for sending emails from local user accounts to "external" mail adresses is postfix and bsd-mailx

If you want to give it a try: sudo apt-get install postfix bsd-mailx.

If you do not want to receive any emails on this host, you could make Postfix a little more secure by binding it to localhost. Run sudo postconf -e 'inet_interfaces = localhost' && sudo service postfix restart.