Mail server can no longer send mail.
 

I've been using Postfix as the MTA on my server for quite a while. I've never had a problem with it. I have it configured with Usermin so that I can use the web gui to read and send emails. I haven't changed anything in an configuration files since I last had it working. The server will still receive emails sent to it, but it can no longer send emails anywhere else.

What's going on here and how do I fix it?

Thanks.

Check with your Internet provider to see if they have blocked/filtered the common SMTP ports. This is common (but misguided) practise amongst Internet providers to eliminate the possibility of people setting up spam relays on their network.

That's what my first thought was, but I was really hoping that wouldn't be the case. I guess I'll have to contact my ISP.

What I forgot to mention last night was you can also try testing for blockades yourself, by using Shields Up from GRC.com, also a nice tool for checking your own firewall.
But the only surefire way to determine blocked ports on the ISP side is by contacting them really, because if they use stealthing techniques it's pretty confusing to determine a true blockade without solid evidence.
My ISP filters all SMTP traffic, the only way I am able to send any mail is solely through their mailserver. Very inconveniant for accounts that won't accept foreign relays.

I did a scan for port 25 with the link you provided and it said it was open.

Alright, then we'll have to check more locally. Have you found any error messages in your postfix/system logs? Have you checked the postfix configuration files for any recent changes you haven't made yourself? (perhaps an automatic package/security update messed up your configuration?)

In my maillog, I have a lot of:
And then there was a:
I've never looked at any mail logs before (I'm a newbie to mail stuff), so I really don't know what those mean.

Your server is telling you it can't find the host it wants to connect to because of an error in name resolution (DNS). So you might want to check this bit of your server configuration, and perhaps your /etc/hosts file for correctness. Is this server on a dynamic IP or a static IP?
No other quirky behaviour other than not being able to send mail outwards?



I'll outline this bit of the log for you:
A machine known by IP address 219.91.93.108 is attempting to connect to your Postfix server's SMTP Daemon.

The foreign host, 219.91.93.108, is rejected by the SMTP Daemon from sending mail because your Postfix configuration denies mail relaying from foreign hosts (foreign meaning those anywhere else than localhost or those host IP addresses specifically allowed in the allow list).
The error occurred right after the HELO request on the SMTP protocol. The foreign host attempted to HELO, which is the very first beginning of an email, with 24.198.61.114. Typically the address after HELO is your server's address, but in case it's not it was spoofed by the machine that attempted to HELO your server. Right after HELO comes the MAIL FROM command, which you see in your log as 'from=<asjdkfeghghw@msa.hinet.net>', in this case it looks like a falsified address.
Just after MAIL FROM, comes RCPT TO, which you see in your log as 'to=<uu1553@so-net.net.tw>'. Right after all that, the foreign host should send the DATA command after receiving an 250 OK message from your server, but:

The connection to the SMTP Daemon was lost after the RCPT TO command from the foreign host. after receiving the error message (Relay access denied) from your server. Since the connection to the SMTP Daemon was cut here, no message DATA was ever received from the foreign host.

The foreign host has disconnected completely from your server.

It looks to me like your server successfully rejected a spam message from being sent.

Well, this is all interesting. Just after I posted those log lines, I realized that I am having a DNS issue. You said that it looked like my mail server successfully rejected spam, but I've noticed that it seems to be more than that.

At the time I first posted this thread, I could receive mail, but not send it. Now it appears I can not receive mail either. I tried to send an email to myself from another account and it did not get through, however, I have multiple domains to this server, so I tried another domain, the other domain worked perfectly.

I can't figure out what my DNS issue is, but I would imagine that I can't send mail because of this DNS issue, and I can't receive mail because of something with the DNS entry of that domain??

Thanks for the help so far, Xolo.

You're welcome.
If you want to pinpoint your DNS problem you could start by making a small note for yourself and find out which domains are broken and write that down. Then the next step i'd take is verify the configuration for each domain, such as the IP address that is associated with each domain. Check if the hosts that belong to the IP addresses are in fact reachable from your server, if not try a different machine to make sure it's not just your server that can't reach them.
From there, you have two choices:
1. You'll have to find out why a host is unreachable if it can't be reached from two different machines.
2. You'll have to figure out why a reachable host can't be connected to from your server.

1 might be tricky if you don't have access to the host itself, but contacting it's owner might be helpful when possible.
2 could be a wild goose chase, but since this is a DNS issue i'd start with the simplest of your own machine which is your /etc/hosts file. It's where your machine stores some simple entries with pairs of IP/hostnames for localhost and possibly others. Second, if you have something running like Bind (a DNS server package) or TmDNS for example, check the logs for those services as they might contain information about the nonresponsive hosts, and possibly some clues as to what has changed lately to mute your Postfix server.
Is it possible at all for the hosts of the domain names to have changed their configuration lately like switching servers or altering DNS/MX entries om their side?

I think I just figured out why no one can send me emails. I just did a dig <domain> IN MX and it gave me a sub domain that I don't use. I changed the DNS entry, but I can't test it for a while.

Aight, keep us posted :) good luck

Ok, it looks like I don't understand DNS systems at all. Basically, all my sub domains work fine with my mail server (except for sending still), but the domain itself will not work.

I can use user@host.domain.tld, but I can't figure out how to configure everything so that I can mail to user@domain.tld. Is it something I have to configure with Postfix, or my DNS record, or what?

Until probably a couple hours after pointing my new domain to my server, I was able to email user@domain.tld, why doesn't it work now?

Ok, I'm now able to receive email. It was actually something stupid on my part. I'm still having DNS issues that are preventing me from sending mail though.

I've been trying to change the network configuration through Webmin so that it is using a different name server, but it doesn't seem to want to change. Right now it's pointing to a router that the server is behind, I want it to point to a different name server, but it just reverts the configuration right after I restart the NICs.

EDIT: I just edited /sbin/dhclient-script so that it would use the right name server, but it still didn't help.