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Using Plusnet email server as a smarthost

On 2023-11-28, I received notice from my email provider that, very shortly, they would not relay mail for domains other than the one(s) you had purchased. This meant I could no longer use their email server as a smarthoste for the hydrus.org.uk domain.

I also had access to my ISP's (Plusnet) email servers but had never used them. Was it possible to use to Plusnet's servers as a of smarthost? A web search indicated that people were indeed making use Plusnet's servers in that fashion. I therefore changed the EXIM configuration on ash to see if I could get it to work.

Plusnet documentation said that no authentication was necessary if access was from a Plusnet IP address, so that was one thing I didn't need to configure.

My first attempt was a failure:

  2024-01-10 16:35:12 1rNbXr-0007o2-0k => xx@gmail.com R=smarthost
  T=remote_smtp_smarthost H=relay.plus.net [212.159.9.107]
  X=TLS1.2:ECDHE_X25519__RSA_PSS_RSAE_SHA256__AES_256_GCM:256 CV=yes
  DN="C=GB,L=Sheffield,O=Plusnet plc,CN=relay.plus.net"
  C="250 NbXrrDVo26TVUNbXsr7oO5 mail accepted for delivery"
  2024-01-10 16:35:12 1rNbXr-0007o2-0k Completed
  2024-01-10 16:35:13 1rNbXt-0007o6-1Y rewrite: missing or malformed local part
  2024-01-10 16:35:13 1rNbXt-0007o6-1Y H=avasout-ptp-002.plus.net [84.93.230.235]
  X=TLS1.2:ECDHE_X25519__ECDSA_SHA256__AES_256_GCM:256 CV=no
  F=<> rejected after DATA:  header syntax (missing or malformed local part:
  failing address in "From:" header is: <>)

Hmm, what was the cause of this? After some reading around, I noticed this comment in this Debian wiki entry:

6. The SMTP server may reject mail without a proper "From:" address. Add a line to /etc/email-addresses to link each linux user to an email address e.g.

  root:sysadmin@example.org

Creating this file with the appropriate user/email mapping worked!

There is another solution that avoids creating the /etc/mail-addresses file: that is, add another rewrite rule to the REMOTE_SMTP_HEADERS_REWRITE macro in /etc/exim4/exim4.conf.localmacros:

  REMOTE_SMTP_HEADERS_REWRITE = *@*.hydrus.org.uk  $1@hydrus.org.uk  f : \
  !*@* $0@hydrus.org.uk f

I also had to update the SPF TXT record in the DNS for hydrus.org.uk to let Google (and others?) know that plus.net was a valid relay.

  "v=spf1 include:plus.net ~all"
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