Mails sent from Tiscali to GMX not received

[GlennLRJ]

Mails sent from my @Tiscali.co.uk address to my @gmx.co.uk address are not received at the latter, with no delivery failure message.
Mails sent from my @Tiscali.co.uk address to another @gmx.co.uk address are also not received at the latter.
In my gmx account, the tiscali address is not in the Blocklist and IS in the Allowlist. Mails are not sent to junk.
Mails from my @gmail.com address are received at gmx OK, as are mails from many other senders.
Mails sent from @gmx.co.uk to @Tiscali.co.uk are received OK by tiscali, but subsequent replies to these mails from tiscali to gmx are not received by gmx.
The test messages are simple plain text with no links or attachments.
This problem has been occurring since at least 5-Aug-24. Two other tiscali users have the same problem, as posted on this forum (see "Emails to GMX not being delivered and no bouce back")
It's not clear if this is a sending (tiscali) or receiving (gmx) problem.

[GlennLRJ]

Copied (& edited) from https://community.talktalk.co.uk/t5/Email/Delivery-Status-Notifications-not-received/m-p/3062397

I am trying to investigate a problem with sending mail from my tiscali address to GMX addresses.

Mails are not received at GMX and no delivery failure notification is received, aka bounce message.

GMX will not investigate unless they can examine the bounce message.
Using Mozilla Thunderbird on a desktop Windows PC, I can request (per message) a Return Receipt (RR) and a Delivery Status Notification (DSN). I understand that the Return Receipt could (if desired) be sent by the person receiving the mail. I assume that the DSN should be sent automatically by the transmitting Email system.
I have tiscali, GMX and Gmail accounts.
When sending a plain text test message I get the following results:
1. from tiscali (via Thunderbird) to GMX : no DSN; mail not received
2. from tiscali (via Thunderbird) to Gmail : mail received OK, but no RR requested *1
3. from tiscali (via Thunderbird) to a wrong GMX address : no DSN (why not?)
4. from Gmail to a wrong GMX address : got DSN OK from Mail Delivery Subsystem <mailer-daemon@googlemail.com>
5. from tiscali (via Thunderbird) to a wrong Gmail address : no DSN (why not?)
6. from tiscali (directly from TalkTalk webmail) to GMX : no DSN; not received

*1 OK, for Gmail, "Read receipts are only available for work or school accounts, not personal Gmail accounts"

Question: Why is the tiscali mail service not generating Delivery Status Notifications when sent mails are not successfully received by the destination? (DSNs are not in Junk / Spam)

[GlennLRJ]

Hi Chris,

Thanks for making contact.

Checked again this evening - mails sent from my tiscali.co.uk address are not received at my gmx.co.uk address, whereas mails sent from my gmail address are received OK.  From Thunderbird I requested both a Read Receipt and a Delivery Status Notification, but got neither.  The missing DSN is a problem, because GMX won't investigate without a 'bounce message'.  Please see separate topic : 

https://community.talktalk.co.uk/t5/Email/Delivery-Status-Notifications-not-received/m-p/3062397

Any help or advice would be appreciated.

Thanks,

Glenn

[Chris-TalkTalk]

Hi Glenn,

How are you getting on, are the emails still not getting through?

Chris