Microsoft engineers are struggling to fix a seven-day-old, self-inflicted Office 365 IMAP outage.

IMAP access to Office 365 tanked on January 18, meaning customers could not access emails using Exchange Online via IMAP or connect third-party mail clients via IMAP.

Microsoft told disgruntled Office 365 customers that the problem affected a limited number of licensees – but that those customers hit had a "large number of users.”

The culprit was found to be a botched Microsoft update that stopped the IMAP protocol automatically loading data from Exchange Online databases.

Following preliminary analysis of the cause, Microsoft told disgruntled users on January 22:

As part of our efforts to improve service performance, an update was deployed to a subset of code which is responsible for obtaining the subscribed folder list. However, the update caused a code issue that prevented the list from being automatically loaded.

Microsoft promised to fix the problem by January 23 – five days after the outage. Those plans then had to be pushed back. Restoration time for the service was estimated for today (January 25), however, Microsoft is still nowhere near a fix.

Meanwhile, one Reg reader got in touch today to complain that all mail sign-ins – including Exchange protocol – had gone titsup.

Microsoft, meanwhile, told users: “The deployment process is taking longer than initially expected due to a configuration issue.

“Engineers reconfigured the problematic portion of capacity and are continuing to monitor the deployment to ensure it propagates throughout the infrastructure in a timely manner. Customers will begin to experience service restoration as the deployment progresses.” ®