Thousands of BT customers faced email meltdown after a Yahoo security update went wrong.
The glitch affected customers who use their own domain names to send email.
Yahoo, which runs email servers for BT, put into place anti-spamming measures which basically blocked any emails that did not have a matching BT/Yahoo address in the From: field.
Certain customers found, as a result, that they couldn't send emails and were sent an error message asking them to follow a link to "validate their domain".
Both BT and Yahoo are keeping schtum as to how many customers were actually affected but the figure is thought to be in the thousands.
In the meantime, the upgrade has been removed but BT has warned that it will be rolled out again.
BT says, however, that the problem is not the filter itself but the unexpectantly high numbers of its customers who use their own domain names.
BT apologised for the confusion, while Yahoo has set-up a web page to clarify the process.
BT added: "Protection of our customers is paramount".