The Wiert Corner – irregular stream of stuff

I’ve a bunch of secondary MX servers using postfix (which I like a lot over sendmail). Basically all their configurations are very similar:

1. To the file /etc/postfix/relay, add a list of domains to relay for, each ending with a space followed by OK as per
   [WayBack] Configure Postfix to relay mail to multiple internal mail servers w/different domains
2. Run postmap /etc/postfix/relay to update the relay database file.
3. Ensure that /etc/postfix/main.cf has these settings (note that the FQDN – in the example smtp3.example.org – isn’t always returned by hostname --fqdn, see below):
   1. inet_interfaces = all
   2. myhostname = smtp3.example.org
   3. smtpd_helo_required = yes
   4. smtpd_helo_restrictions = permit_mynetworks, reject_invalid_helo_hostname, reject_non_fqdn_helo_hostname, reject_unknown_helo_hostname
4. rcpostfix restart && rcpostfix status

Check the functionality with journalctl -u postfix.

FQDN – how to get myhostname

This usually gives a good indication of your external FQDN, but depending on your network circumstances it might not at all be the FQDN of your machine:

dig +noall +answer +short -x `curl -s ipv4.whatismyip.akamai.com` | sed 's/\.$//'

I got there through these StackExchange answers:

• [WayBack] linux – How do I get cURL to not show the progress bar? – Stack Overflow
• Reverse DNS lookup through dig: [WayBack] 10 Linux DIG Command Examples for DNS Lookup and [WayBack] linux – Bash: Reverse DNS Lookup of Active IP Addresses – Super User
• remove the final . from the dig answer: [WayBack] text processing – Remove last character from line – Unix & Linux Stack Exchange

Testing with sendEmail

Then test with sendEmail from [WayBack] GitHub – mogaal/sendemail: lightweight, command line SMTP email client, with statements like these

The most recent version is now at [Wayback/Archive.is] GitHub – zehm/sendEmail: SendEmail is a lightweight, command line SMTP email client.

• sendEmail -o fqdn=sending.example.org -f sender@example.org -t recipient@example.com -u message-subjetc -s smtp3.example.org -m message-text
• sendEmail -o fqdn=sending.example.org -f sender@example.org -t recipient@example.com -u message-subjetc -s smtp3.example.org -m message-text -v -v -v -v

If you get an error containing 450 4.7.1 … Helo command rejected: Host not found, then reject_unknown_helo_hostname works, but your -o fqdn= parameter contains an invalid hostname.

More sending examples are in the sendEmail documentation.

If you want to know more about fighting SPAM, then continue at [WayBack] Fighting Spam – What can I do as an: Email Administrator, Domain Owner, or User? – Server Fault

–jeroen