BT email clients receiving mailbox full NDR when sending email to my client whom was using gmail with own .co.uk domain. gmail mailbox was around 50% full. no forwards were setup within gmail account. problem only affected BT customers, all other email from other ISP's was received without issue.
BT Clients could email other @gmail.com accounts but not this gmail account with .co.uk domain.
Turns out BT hadn't properly deleted the authoritative domain correctly when my clients .co.uk domain was transferred out of BT 2 years prior.
You can only imagine how the support call went with BT screen reading robot tech support (client no longer BT customer, .co.uk domain transferred out 2 years prior, no BT broadband, no BT land line). Was done in little over an hour!! I couldnt get a quick email confirmation from BT tech support though as "they dont have email". Can you believe IT support doesn't have access to email!!
hesl03uker HEMV4AUKER.he.local are btconnect mail servers
inbound ibmr.btconnect.com
To prove:-
Telnet to BTconnect email server
try and send email as problem email address
if it accepts email it could be cause
if it relay denied then that isnt issue