554 Message rejected for policy reasons

[fwbluem]

Whenever I send a message from my Gmail account to a btinternet address, it bounces back with the error:

Diagnostic-Code: smtp; 554 Message rejected for policy reasons (2.1.2.3)

The problem occurs when using Thunderbird, Mac Mail or webmail and is not specific to my own Internet connection.

I have tried adding my Gmail address to the 'safe senders' list of my BT address but this has not helped.

It would seem my Gmail address is on some sort of BT blocklist.

Please can someone help me get this sorted out.

[fwbluem]

I've got multiple Gmail addresses, but only one will not send to BT addresses. What could be the problem?

[fwbluem]

I have seen on similar threads that contacting the postmaster can help (https://www.bt.com/help/email/fix-a-problem/sending-or-receiving-email/bt-email---best-practices-for...), but it does say at the bottom of that page that 'the postmaster team will not respond to queries raised by  BT consumer customers who report send/receive issues', which of course I am!

Does anyone know if this is still the correct thing to do?