k9 mail certificate error - sending to relay.plus.net

[#1]

Hi - I have been using K9 email client on phone/tablet for yonks with zero problems, but sending from K9 now gets certificate error (CertPathValidatorException - trust anchor for certification path not found) so don't know if this is something changed in K9, or something has changed at relay.plus.net

There are no problems sending to relay.plus.net using Outlook on the PC

The K9 settings which have worked OK until today have StartTLS; port 587; Normal Password; Username; Password and Client certificate = None (no other options provided)

If I go through the config settings in K9 it now gives this message

Any ideas...?

[#2]

@geoffers Which phones tablets - make a model? Sounds like a known issue.

[#3]

@jab1 

can you confirm if you can see the image please? A thanks will be enough

[#4]

@Dan_the_Van I  can now, but couldn't when I posted my initial reply.

[#5]

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@jab1 wrote:

@geoffers Which phones tablets - make a model? Sounds like a known issue.

---

John,

Sounds similar to the known issue, but is not the same as the known issue.

The known issue relates to INBOUND sever certificates and the failure of some device vendors to supply CURRENT G5 root certificates to their users.

This issue relates to OUTBOUND server certificates which look as though they were installed today ... and DO NOT use the G5 certificates...

PS C:\Users\kevin> curl.exe -w '%{certs}' smtps://relay.plus.net
214-This is smtp ESMTP service help
214-To contact postmaster send email to postmaster@plus.com.
214-For local information send email to Postmaster at your site.
214 end of HELP info
Subject:1.3.6.1.4.1.311.60.2.1.3=GB, 2.5.4.15=Private Organization, serialNumber=03279013, C=GB, L=Sheffield, O=Plusnet plc, CN=relay.plus.net
Issuer:C=US, O=DigiCert Inc, CN=GeoTrust EV RSA CA G2
Version:2
Serial Number:06:ad:2c:f1:7b:de:0a:7e:2f:8c:b8:09:4a:b9:01:81:
Signature Algorithm:sha256WithRSAEncryption
Start Date:2025-08-15 00:00:00 GMT
Expire Date:2026-08-14 23:59:59 GMT

We have flagged the issue to the business.

[#6]

Cheers, @Townman . You can tell I'm a little out of touch.😀

[#7]

@jab1 - Thanks: Phone is OnePlus9 running android 14
Tablet is Lenovo P11 Pro running android 11

[#8]

@jab1 

Nah John,

It is not a matter of being 'out of touch' but discerning detail.  There are so many bullets in the air, that there is a danger of seeing them as all being the same.  INBOUND has been around all week; this OUTBOUND issue is new, though why is not clear.

We have this escalated.

[#9]

@Townman and @jab1  - don't know if this helps, but I just pulled out an old Motog5 phone which is sending emails OK from k9 - however in this phone's SMTP setting it had Security=None whereas the OnePlus & Lenovo settings has Security=StartTLS

[#10]

@geoffers 

That does not in the sense you indented help: The issue is connected to the encryption security certificate.  If you use the SMTP server without encryption, you will not encounter the issue with the certificate.

[#11]

@geoffers More @Townman 's field of expertise than mine, so I'll keep quiet 😁. The issue has been noted, though.

[#12 · Accepted solution]

@geoffers 

Can you please give it another go - the certificates have been updated.

[#13]

Brilliant - looks like that fixed it, thanks 😎

[#14]

indented

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