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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘Internet protocol suite’ Category

Common SMTP message size limits

Posted by jpluimers on 2020/04/08

After a 2018 discussion with a “zorgkantoor” (Dutch for office that arranges for special long term health care needs, successor of AWBZ) about their very low (10 megabyte) SMTP message size limit – even though they expect scanned PDF documents.

Their web-care team posed this limit as normal, so I made a list of limits in their peer group, common world-wide and well-ranked Dutch internet providers.

My plan is to check the progression of these limits over time.

Note these are the bruto message sizes including encoded attachments. Since encoding in [WayBack] MIME Base64 – Wikipedia has a overhead of at least 37% (encoded size is at least 1.37 the original size), the unencoded maximum size is less than 73% of what is listed below.

References:

2018

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Posted in Communications Development, Development, eMail, Internet protocol suite, Power User, Python, Scripting, SMTP, SocialMedia, Software Development, TCP | Leave a Comment »

mail.com (handled by gmx.com) can reject SMTP connections as early as in the EHLO stage

Posted by jpluimers on 2020/04/06

I found this in one of my logs a a while ago:

Error when executing EHLO command for domain mail.com on SMTP server mx00.mail.com.
(554, 'mail.com (mxgmxus007) Nemesis ESMTP Service not available\nNo SMTP service\nBad DNS PTR resource record.\nFor explanation visit http://postmaster.gmx.com/en/error-messages?ip=37.153.243.242&c=rdns')

It means that gmx.de / mail.com are among the strictest email handling providers I know. I don’t blame them: EHLO is at the start of an extended SMTP session.

At [WayBack] Error messages | GMX Postmaster it indicates:

5xy Bad DNS PTR resource record

Emails from your email server were rejected because the PTR Resource Record (PTR-RR) of your IP address does not follow our guidelines. Possible reasons for this can be:

–jeroen

Posted in Communications Development, Development, eMail, Internet protocol suite, Power User, SMTP, SocialMedia, TCP | Leave a Comment »

Determine actual message size limit when you only get “552 5.3.4 Message size exceeds fixed limit”

Posted by jpluimers on 2020/03/26

Often when you send large emails the only  reply you get is a non-descriptive message like 552 5.3.4 Message size exceeds fixed limit from the SMTP server without an indication what the limit actually is.

Most SMTP servers however implement extensions in the EHLO greeting that returns a SIZE mail parameter. You can query it by hand using this:

telnet aspmx.l.google.com smtp
Trying 108.177.119.27...
Connected to aspmx.l.google.com.
Escape character is '^]'.
220 mx.google.com ESMTP 32si3005669edb.510 - gsmtp
EHLO example.org
250-mx.google.com at your service, [80.100.143.119]
250-SIZE 157286400
250-8BITMIME
250-STARTTLS
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8
QUIT
221 2.0.0 closing connection 32si3005669edb.510 - gsmtp
Connection closed by foreign host.

There you can see the maximum message size at the time of writing is 157286400 bytes which is about 150 megabytes.

There is a nice Python script showing how to obtain it at [WayBack] Getting Information from EHLO | Erle Robotics Python Networking Gitbook Free (note this one does send an email, so you might want to trim the example if you just want to see the size).

More background reading:

Trimming down the Python script so it queries message size for each mail server of a domain

This turns out to be a tad more complex, because DNS functionality isn’t part of core Python, and the rdata part of DNS records ends with a dot, which might not be usable with the SMTP library.

References for me when trimming down:

–jeroen

Posted in Communications Development, Conference Topics, Conferences, Development, Event, Internet protocol suite, Power User, SMTP | Leave a Comment »

Linux – How to Securely Copy Files Using SCP examples

Posted by jpluimers on 2020/03/16

I love short and to the point examples. The list of permutations for scp is at [WayBack] Linux – How to Securely Copy Files Using SCP examples.

–jeroen

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Posted in *nix, *nix-tools, Development, Internet protocol suite, Power User, SSH, TCP | Leave a Comment »

ssh – Why OpenSSH deprecated DSA keys – Information Security Stack Exchange

Posted by jpluimers on 2020/03/10

In a lot of ssh-keygen related posts, you still see DSA being mentioned, though that has been deprecated and later removed from OpenSSH.

I wondered why, so I did some digging.

TL;DR: it’s complicated:

  • different standards mandating eventually conflicting parameters,
  • extending the parameters would require protocol extension,
  • a logjam vulnerability for certain combinations of parameters and finally
  • better algorithms having become available.

Some of the related topics cannot be archived in the WayBack machine or refuse being archived at Archive.is, so here is a list of partially archived relevant links:

–jeroen

Posted in Communications Development, Development, Internet protocol suite, Power User, Security, SSH, TCP | Leave a Comment »

 
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