topic Re: Email send fails / SMTP Certificate in Email

Email send fails / SMTP Certificate

PhilipHeyes — Thu, 20 Aug 2026 07:52:45 GMT

Are you are seeing email setup or send issues that come out out of the blue . . .

I am seeing an SMTP Certificate rejection with Blue Mail app on Android when Certificate Check is enabled for either STARTTLS ( 587 ) or SMTPS ( 465 ). Sadly I am not getting any detailed explanation.

The Certificate rejection by Blue Mail occurs on the following SMTP servers that all appear to share
the Plusnet Certificate of relay.plus.net ( see CheckTLS attached ) :

Plusnet hosted relay.plus.net

Greenby hosted relay.force9.net |

Greenby hosted relay.metronet.co.uk

Greenby relay.metronet.co.uk is also named on this Certificate, but not affected, it lacks STARTTLS & SMTPS.

I was about to raise Greenby Ticket on this, but the issue does not affect versions MS Outlook or Thunderbird installed on MS Windows and it is not affecting Samsung Email app on Android, all are sending perfectly via relay.force9.net

My workaround is to use SMTP mail.enmail.co i.e. to avoid a Certificate that was issued to Plusnet & with this I am back in business with Blue Mail. Other options may be to disable Certificate check or Disable encryption, if permitted.

Quite a puzzle, what has worked well with relay.force9.net for 6+ months has ground to a halt in just one app.

Re: Email send fails / SMTP Certificate

MisterW — Thu, 20 Aug 2026 10:18:54 GMT

Is it a missing intermediate certificate ?

Re: Email send fails / SMTP Certificate

PhilipHeyes — Thu, 20 Aug 2026 10:44:26 GMT

The start of the 2nd Certificate is obscured by the page header in the PDF.

Certificate #2 of 3 (sent by MX):
Cert VALIDATED: ok
cert not revoked by OCSP

Data:

Version: 3 (0x2)

Serial Number: 0e:a7:1a:b4:b9:b3:0f:67:f5:a6:e6:31:76:9d:ce:9c

Validity:

Not Before: May 26 00:00:00 2022 GMT

Not After: May 25 23:59:59 2032 GMT

Re: Email send fails / SMTP Certificate

MisterW — Thu, 20 Aug 2026 11:01:56 GMT

Not my area of expertise, but Yes, the certificate chain is correct but AIUI devices store root & intermediate certificates in their certificate store.

The fact that some devices work and some don't MIGHT indicate that the device that fails doesnt have one of the root/intermediate certs in its store and so cant completely validate the certificate chain

Re: Email send fails / SMTP Certificate

PhilipHeyes — Thu, 20 Aug 2026 11:20:47 GMT

I emailed a detailed question to Blue Mail support.

The bit that looks to have changed in my environment is the last update for Blue Mail app was on 15th of this month.