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Old 02-27-2008, 09:17 PM   #1
Registered: Jan 2008
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sendmail errors: can't resolve name, user unknown

Hi all, i have a problem with my sendmail in one of my linux boxes. To add to the woe, i am not familiar at all with sendmail. Let me explain a little bit about our setup.

Previosuly it was working fine wihtout firewall, yes i know the first culprit that pops into the mind is the firewall but i have allowed ports SMTP and port 110 from external (untrust zone) to internal (trusted zone) and from trusted zone to untrusted zone, the policy is ANY. I don't know what else i should allow. Let's put firewall aside first. On the mail setup, my servers do not belong to any domain, and we do not have an internal domain name server but we point to an external DNS. When i ping, that name can be resolved. Let's say my web server is called www-04 and i want to send to another web server in the same segment called www-03 to a user called sy. If i execute a command of mail sy@www-03, the error message is unknown user. Am i supposed to put the web servers in some domain? Can it work without any domain? What basic setup should there be for the servers. I can't send out to another domain called too. For example, if i want to send to, it says connection refused or the domain name is invalid. ANy idea? Any suggestions?
Old 02-29-2008, 08:01 AM   #2
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There's different ways to go about this but the basic rule of thumb is that if you want to send and receive e-mail you must be "known" to somebody, that's how the protocol works. For "simple" e-mail routing between only servers in a LAN you can do with static host mappings (or a simple DNS server that advertises those records) and if you want to send e-mail to the outside you can use your ISP's MTA.

If you want to manage, send and receive e-mail for the domain yourself you'll have to (have your domain registrar) set up MX records that point to one or more e-mail servers within your domain as receiving MTA's. This could be a shared box in your ISP's colo or one on your own premises (configure a secondary or fallback one in another location if your ISP doesn't provide you with those services). It can be a box that is attached to a LAN (not good) or an isolated bastion host (DMZ) that relays e-mail through to other MTA's inside a LAN segment (way better).

Sendmail isn't that hard to configure but if you'll be the one to configure and admin the e-mail at your company you'll have to familiarise yourself with at least the basics. Luckily theres lots of documentation around to help you here at, TLDP and probably Howtoforge as well and of course O'Reilly's excellent "Bat Book". Once you've read a few you'll be able to see why it's not working and ask more specific questions on how to fix things. Depending on your level of technical knowledge you may find this all a bit much. Maybe reading Quick HOWTO : Ch21 : Configuring Linux Mail Servers could be a gentler introduction.


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