Emails silently discarded

[GlennLRJ]

I normally receive plenty of emails to a tiscali.co.uk address, but I've discovered that occassionally some emails are not received. They are not moved to Junk, either on the client side (Thunderbird) or at webmail level (TalkTalk). The sender does not receive a failure-to-send notification. The not-received mail can be successfully forwarded to me by another recipient, so it is not being rejected because of content.
Are there circumstances where mails are discarded silently by the TalkTalk server? What else is happening?

[GlennLRJ]

@Gondola Many thanks for the information.

[Gondola]

The mail from ukshowchoir.com that includes the Return-Path: with any ukshowchoir.com email address will be treated as unauthenticated. Whoever manages that domain must add a valid SPF text file to the DNS Name Servers in order to improve deliverability.

Other mail that includes a Return-Path: address for the sendgrid.net domain is fully authenticated by dint of the SPF file that the bulk mail sending service Sendgrid uses. Sendgrid is a marketing email company.

The Return-Path: email address serves two purposes. It's used by TalkTalk to pick up the SPF file from the Sendgrid Name Servers to check that the marketing mail is authentic and being sent by permitted mail senders.

The other use, in this case, is by Sendgrid in the event that mail is bounced back as undelivered. Sendgrid will know from the email address what customer is involved and what the recipient email address was. In this case your tiscali email address that I've obscured from your post. On getting this bounceback after retrying Sendgrid will then suppress sending to your email address as a protective measure. This stop on sending is best practice for bulk mail services / marketing mailshots in order not to send to mailboxes not accepting or rejecting delivery. The stop on sending is to avoid the risk of being placed on spam blacklists.

[GlennLRJ]

@Gondola Thanks again for the reply.
I cannot check the header of the mail I didn't receive. The forwarded version has the forwarder's Return-Path: ...@yahoo.co.uk.

A 1-to-1 email from mail@ukshowchoir.com successfully received today has Return-Path: <mail@ukshowchoir.com>
Another mail successfully received on 5-Sep has Return-Path: <bristol@ukshowchoir.com>
Doesn't that suggest that an SPF file already exists for ukshowchoir.com?
However, other mails recently received from mail@ukshowchoir.com have:
Return-Path: <bounces+31891978-4fe1- obscured for customer security  @sendgrid.net>
What does that imply?

[Gondola]

If the Return-Path email address is also in the email header as the ukshowchoir.com domain then there is no SPF file set up for that domain. No SPF file and no DMARC policy means that TalkTalk will treat the email as unauthenticated.

If you look at the raw header of the original email / view source and identify the line, usually at or near the top of the header, that's titled Return-Path: then that will have an email address for authentication purposes. If that email domain is ukshowchoir.com then the administrator for that domain needs to add an SPF text file to the DNS Name Servers so that the TalkTalk inbound servers can check the sending IP address is authorised to send mail for that domain.

Let me know if the Return-Path: address is something different and I'll check that for you.

[GlennLRJ]

Thanks, Gondola, for the reply.
There are no filter rules set up either in my mail client (Thunderbird) or in TalkTalk webmail. The most recent known missed mail was a mail-out from mail@ukshowchoir.com. I'm not sure if that counts as a 'vanity' address. Others received the mail from this address successfully, so why would TalkTalk regard it as not authenticated? The sender can see that my mail was sent but not 'opened'.

[Gondola]

I would check your mailbox to ensure there are no filter rules defined that might be affecting mail that would otherwise arrive in your online Inbox.

TalkTalk would reject mail that is not fully authenticated. Are these 'missing' mail messages from an individual that has a personal domain / vanity email address?