We have had some issues with users using a @btinternet.com email accounts.
If they reply to a case message, they get an undeliverable message and we obviously don’t get the incoming email as it has not been delivered.
Has anyone experienced similar issues with this domain or any others?
Any ideas on what to recommend the customer, as we don’t want to ask them to use another email account if possible?
Many Thanks,
JonathanFS (EHDC)
Did the notification about the message being undeliverable specify why? Unfortunately it sounds like they might need to contact their email provider.
Unfortunately, this is way to technical to ask our customers.
The problem is 100% their end.
I may ask the customer to add info@central.selfservice.easthants.gov.uk to their contact list to see if that would help.
As It feels like over zealous spam filtering 
Could you ask them to forward the email about the delivery failure to you directly? Or see if there’s someone in IT with a btinternet email that could help you troubleshoot?
It’s definitely really difficult when the issue is impacting our users but isn’t a fault with our systems.
It appears this may be part of a wider issue affecting BT domains, with several organisations reporting that emails to BT domains are being rejected globally.
Could this be an issue on their side? There are reports suggesting they received communications earlier in the year advising changes to routing records, which may not have been implemented.
I suggest using MXToolbox to check the routing domains, as this should help identify any invalid records or routing issues.