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Archive for the ‘SMTP’ Category

Cryptosense Discovery

Posted by jpluimers on 2022/03/15

This is cool: [Wayback] Cryptosense Discovery:

Free tool that discovers security configuration errors in SSH and TLS servers and explains how to fix them. Supports STARTTLS and can also scan HTTPS, POP3, IMAP and SMTP servers.

It gives you a list of servers a target domain uses (for purposes like web, email, etc) that can have external encryption enabled, then allows you to test these.

The list by default has only servers within that target domain enabled, but you can optionally include other servers (for instance if a domain uses a third party for their SMTP handling).

Basically it is the web-counterpart of a tool like (which I have written about before).

Found while checking out how to test the MX security of a domain using [Wayback] as I forgot the syntax, which in retrospect is dead easy as per [Wayback] tls – How to use on an SMTP server? – Information Security Stack Exchange (thanks [Wayback] Z.T.!): --mx <domain name>

works fine. -t smtp <ip>:25

and -t smtp <ip>:587

also work fine.

Note that not specifying the port assumes port 443, despite specifying protocol smtp. That doesn’t work.

Also, you might try which does the same thing only better

That website is made by the cool people at [Wayback] Cryptosense.

Both are a lot easier than the alternatives described in [Wayback] Blog · How to test SMTP servers using the command-line · Halon MTA: using nslookup and dig for determining the affected hosts, using nc or telnet for testing basic connectivity, using [Wayback/] openssl s_client to test TLS, and [Wayback/] smtpping for measuring throughput.

In addition to the above tools mentioned in the blog, I’ve also used sendEmail (note case sensitivity), ehlo-size, and swaks.

This is what I tested:


Posted in *nix, *nix-tools, Awk, bash, bash, Communications Development, Development, DNS, Encryption, grep, HTTPS/TLS security, Internet, Internet protocol suite, Power User, Scripting, Security, SMTP, Software Development, SSH, ssh/sshd, TCP,, TLS | Leave a Comment »

Which SMTP Port Should I Use? Learn Ports 25, 465, & 587 (and unofficial port 2525) | Mailgun

Posted by jpluimers on 2021/11/30

When trying to deliver mail, it is important to know which protocols and ports you can use.

On smtp, smtp-submission, smtps (ports 25, 587 and 465) and unofficial port 2525 (which Maingun maps to `smtp-submission): [Wayback] Which SMTP Port Should I Use? Learn Ports 25, 465, & 587 | Mailgun

Quote on why smtps port 465 is hardly used:

Port 465:

IANA has reassigned a new service to this port, and it should no longer be used for SMTP communications.

However, because it was once recognized by IANA as valid, there may be legacy systems that are only capable of using this connection method. Typically, you will use this port only if your application demands it. A quick Google search, and you’ll find many consumer Inbox Service Providers’ (ISPs) articles that suggest port 465 as the recommended setup. However, we do not recommend it, as it is not RFC compliant.


Posted in Communications Development, Development, Internet protocol suite, SMTP, Software Development, TLS, Web Development | Leave a Comment »

Some scripts and tips for easing the maintenance of a postfix based SMTP system

Posted by jpluimers on 2021/11/24

A few scripts and tips I found Googling around.

Deleting queued messages by regular expression pattern

I have seen the below script numerous time, usually without any attribution (for instance [Wayback] Postfix Flush the Mail Queue – nixCraft and  [Wayback] – Following script deletes all mail from the mailq which matches the regular expression specified as the first argument · GitHub).

The earliest version I could find was in [Wayback] ‘Re: delete messages from mailq’ – MARC by [Wayback] ‘Ralf Hildebrandt ‘ posts – MARC:

--- snip ---

$REGEXP = shift || die "no email-adress given (regexp-style, e.g. bl.*\!";

@data = qx</usr/sbin/postqueue -p>;
for (@data) {
  if (/^(\w+)\*?\s/) {
     $queue_id = $1;
  if($queue_id) {
    if (/$REGEXP/i) {
      $Q{$queue_id} = 1;
      $queue_id = "";
#open(POSTSUPER,"|cat") || die "couldn't open postsuper" ;
open(POSTSUPER,"|postsuper -d -") || die "couldn't open postsuper" ;

foreach (keys %Q) {
  print POSTSUPER "$_\n";
--- snip ---

And then use:
% delete-from-mailq "^test"



[Wayback] How do I check the postfix queue size? – Server Fault

Lots of great answers and pointers to useful guides/software there.


[Wayback] Postfix Bottleneck Analysis points to [Wayback] Postfix manual – qshape(1): qshape - Print Postfix queue domain and age distribution, then explains about different scenarion and queues:


postqueue -p | tail -n 1

Last line in the postqueue -p shows how many requests and size:

-- 317788 Kbytes in 11860 Requests.

View queues size

I tried finding the original posting of the below script, but could not. If you find it, please let me know.

#!/usr/bin/env perl

# postfix queue/s size
# author: 
# source:

use strict;
use warnings;
use Symbol;
sub count {
        my ($dir) = @_;
        my $dh = gensym();
        my $c = 0;
        opendir($dh, $dir) or die "$0: opendir: $dir: $!\n";
        while (my $f = readdir($dh)) {
                if ($f =~ m{^[A-F0-9]{5,}$}) {
                } elsif ($f =~ m{^[A-F0-9]$}) {
                        $c += count("$dir/$f");
        closedir($dh) or die "closedir: $dir: $!\n";
        return $c;
my $qdir = `postconf -h queue_directory`;
chdir($qdir) or die "$0: chdir: $qdir: $!\n";
printf "Incoming: %d\n", count("incoming");
printf "Active: %d\n", count("active");
printf "Deferred: %d\n", count("deferred");
printf "Bounced: %d\n", count("bounce");
printf "Hold: %d\n", count("hold");
printf "Corrupt: %d\n", count("corrupt");

Various commands

[Wayback] Inspecting Postfix’s email queue – Tech-G explaining about:

  • mailq
  • postqueue -p
  • postcat -vq XXXXXXXXXX (where XXXXXXXXXX is the message ID)
  • postqueue -f / postfix flush
  • postsuper -d to delete messages

More of these in [Wayback] Postfix Mail Queue Management – Linux Hint and [Wayback] Postfix Bottleneck Analysis: queues.


Based on [Wayback] Using “make” for Postfix file maintenance

MAPS = relays.db aliases.db transport.db relocated.db \
        virtual.db sender_checks.db rejected_recips.db \

all : $(MAPS)

aliases.db : aliases

%.db : %
        postmap $*

This is my Makefile that runs fine on Tumbleweed (note: all 8-space indents are TAB characters):

MAPS =  /etc/aliases.db \
        transport.db \
        virtual.db \
        helo_access.db \
        canonical.db \
        sasl_passwd.db \
        relocated.db \
        relay.db \
        access.db \
        relay_ccerts.db \

all : $(MAPS)

aliases.db : aliases
        @echo "Rebuilding $@."

%.db : %
        @echo "Rebuilding $@."
        postmap $*

In the future, I might try [Wayback] Makefile.postfix · GitHub, though I think it is convoluted:

## Postfix: Makefile to update *.db files
POSTCONF= /usr/sbin/postconf
POSTMAP= /usr/sbin/postmap
default: postmap
postmap: Makefile.postmap
@echo 'Updating database files …'
$(MAKE) -f Makefile.postmap
@echo 'Updating $@ …'
@set -e; \
rm -f $@.$$$$.tmp; \
echo 'POSTMAP=$(POSTMAP)' >>$@.$$$$.tmp; \
echo 'postmap::' >>$@.$$$$.tmp; \
config_directory="$(PWD)"; \
{ $(POSTCONF) -c $(PWD) || kill $$$$; } \
|tr ' ' '\n' \
|sed -n \
-e 's/,$$//' \
-e 's#^hash:\$$config_directory/##p' \
-e 's#^hash:'"$$config_directory/##p" \
|sort -u \
|while read mapfile; do \
echo "postmap:: $$mapfile.db" >>$@.$$$$.tmp; \
echo "$$mapfile.db: $$mapfile" >>$@.$$$$.tmp; \
echo " \$$(POSTMAP) $$<" >>$@.$$$$.tmp; \
done; \
mv $@.$$$$.tmp $@



[Wayback] Ralf Hildebrandt

Ralf Hildebrandt is an active and well-known figure in the Postfix community. He’s a systems engineer for T-NetPro, a German telecommunications company and has spoken about Postfix at industry conferences and contributes regularly to a number of open source mailing lists.

Co-author of this book: [Wayback: Book of Postfix State-of-the-Art Message Transport ISBN 9781593270018] (which used to have its own site: [Wayback: The Book of Postfix]

Book of Postfix

State-of-the-Art Message Transport

By Patrick KoetterRalf Hildebrandt

Publisher: No Starch PressRelease Date: March 2005Pages: 496

Best practices for Postfix–the popular alternative to Sendmail. Developed with security and speed in mind, Postfix has become a popular alternative to Sendmail and comes preinstalled in many Linux distributions as the default mailer. The Book of Postfix is a complete guide to Postfix whether used at home, as a mailrelay or virus-scanning gateway, or as a company mailserver. Practical examples show how to deal with daily challenges like protecting mail users from spam and viruses, managing multiple domains, and offering roaming access.

This is a great review of the book: [Wayback] The Book of Postfix (Ralf Hildebrandt, Patrick Koetter)


For my postfix studies… « The Wiert Corner – irregular stream of stuff



Posted in *nix, *nix-tools, bash, Communications Development, Development, Internet protocol suite, Makefile, postfix, Power User, Scripting, SMTP, Software Development | Leave a Comment »

Some links on SMTP tar-pit to lessen SPAM

Posted by jpluimers on 2021/11/03

Some links for my archive; note that pure tar-pits by now are also hampering large email sender services like SendGrid, Mailgun and Amazon SES.

So the below links are for educational and historic purposes only.

I assembled these links because out of a sudden, Ring 2FA verification emails could not be delivered any more.

Ring 2FA came mandatory towards the end of February 2020.

Some links on that:

Sendmail timeouts:


Read the rest of this entry »

Posted in *nix, Communications Development, Development, HIS Host Integration Services, Internet protocol suite, Power User, SMTP | Leave a Comment »

On my list of things to try: Amazon SES for outbound/inbound email handling

Posted by jpluimers on 2021/08/10

SES mail servers at the time of writing


# nslookup -type=TXT | grep "v=spf1"   text = "v=spf1 ip4: ip4: ip4: ip4: ip4: ip4: ip4: ip4: -all"I


C:\>nslookup -type=TXT | find "v=spf1"
Non-authoritative answer:
        "v=spf1 ip4: ip4: ip4: ip4: ip4: ip4: ip4: ip4: -all"

These addresses use a compact CIDR notation to denote ranges of networks containing ranges of network IPv4 addresses.

CIRD processing to sendmail access file

(this is linux sendmail only)

Converting the nslookup outout to a CIDR based sendmail /etc/mail/access excerpt goes via a pipe sequence of multiple sed commands:

# nslookup -type=TXT | grep "v=spf1" | sed 's/\(^.*"v=spf1 ip4:\| -all"$\)//g' | sed 's/\ ip4:/\n/g' | xargs -I {} sh -c "prips {} | sed 's/$/\tRELAY/g'"   RELAY   RELAY

What happens here is this:

  1. Filter out only spf1 records using grep.
  2. Remove the head (.*v=spf1 ip4:) and tail ( -all") of the output, see [WayBack] use of alternation “|” in sed’s regex – Super User.
  3. Replaces all ip4: with newlines (so the output get split over multiple lines), see [WayBack] linux – splitting single line into multiple line in numbering format using awk – Stack Overflow.
  4. Convert the CIDR notation to individual IP addresses (as sendmail cannot handle CIDR),
    1. This uses a combination of xargs with the  sh trick to split the CIDR list into separate arguments, and prips (which prints the IP addresses for a CIDR); see:
    2. Alternatively, use
  5. Replaces all end-of-line anchor ($) with a tab followed by RELAY, see

You can append the output of this command to /etc/mail/access, then re-generate /etc/mail/access.db and restart sendmail; see for instance [WayBack] sendmail access.db by example |

Without the xargs, the output would look like this:

# nslookup -type=TXT | grep "v=spf1" | sed 's/\(^.*"v=spf1 ip4:\| -all"$\)//g' | sed 's/\ ip4:/\n/g'



Posted in *nix, *nix-tools, Amazon SES,, Cloud, Communications Development, Development, Infrastructure, Internet protocol suite, Power User, sendmail, SMTP, Software Development | Leave a Comment »

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