That sounds very unsatisfactory, nobody is going to delve that deep, surely
the folk using this need a rethink or at the very least send a bounce.
Brian

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"Martin Brown" wrote in message
...
On 11/02/2021 10:34, Tim Streater wrote:
On 11 Feb 2021 at 10:19:45 GMT, Martin Brown
wrote:

On 11/02/2021 09:30, Tim Lamb wrote:
Anyone else?
3 over the last 10 days.
Messages sent using Thunderbird but said not to be received by the
addressee.
I don't normally tick the *message read* box so there is no warning.

What's this "message read" box business?

When the recipient opens the mail it tries to send a message back to that
effect (no guarantee they have read it though - only opened it and allowed
the message receipt email to escape). I block them.

You are not giving us much to go on. I presume there was no bounce.
First question is are the missing emails in their spam box?
Might the email have been filtered for looking like spam?

Certain SPF rules seem to have been tightened recently which was causing
some people problems if their SPF record was not exactly right. Mainly
when sending emails to other corporate sites with SPF checking.

What ar SPF rules and where are they and who applies them to what?

It is supposed to be an antispam measure. It doesn't really work but it
has gained traction and it catches out the unwary rather too often.

If you check your domain name using nslookup you can see if they are
right. Incantation from a DOSBOX is:

NSLookup.exe -q=TXT nonad.co.uk

Mine looks like this: "v=spf1 include:spf.protection.outlook.com -all"

I'm guessing yours might come back "not found" or "none". The recipient
might well have your email but in some quarantine spammer junk mail box.

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Regards,
Martin Brown