I'm Tech Lead at parkrun and we're experiencing an issue that seems to be affecting an increasing number of our participants with @talktalk.net email addresses.
In each case, your servers are responding with:
smtp; 550 5.1.1 XXXXXXXXXXXXXX Recipient Undeliverable (TT512)
(where XXXXXXXXXXXXXX is a string of letters and numbers - seems to be unique each time)
Our outgoing mail service for these types of emails (AWS SES) regards this as a Permanent bounce so adds the address to a suppression list.
Can you please advice the best way to resolve?
