Many people with a 365 email account who have worked with a company for ages or frequently deal with particularly large emails will have received warnings about their quota being hit soon over the years. Microsoft have a plan to reduce the chances of that happening.
Later this year, Microsoft is introducing a new feature in Exchange Online (which powers 365 email): automatic email archiving based on mailbox size. This change is designed to prevent users from running out of mailbox space and to keep email flowing smoothly, especially in organisations where time-based archiving isn’t quite keeping up with growing inboxes.
Currently, most auto-archiving in Exchange is based on time. For example, emails older than two years might move to an archive mailbox automatically. But with the rise of large attachments, automated messages, and AI-generated content like Copilot summaries, some users are hitting their mailbox limits faster than expected. That’s where size-based archiving comes in.
When a mailbox reaches about 90% of its storage capacity, Microsoft will now trigger an automatic cleanup: the system will begin moving older emails to the user’s archive mailbox until the mailbox drops back below that threshold. Nothing is deleted, and users can still search for and access those messages in their archive. Emails that are specifically marked “never move to archive” won’t be touched, and existing time-based archiving rules will continue to work as they do now.
One important note: this new archiving method only works if archive mailboxes are already enabled. If a user doesn’t have an archive set up, nothing will be moved, and Microsoft isn’t planning to enable archives automatically. In these cases, users will continue to receive the quota warning as they do now. Also, while administrators will be able to see when and how auto-archiving occurs, they won’t be able to disable the feature. Microsoft see it as a built-in safeguard to stop mailboxes from becoming unusable due to overfilling.
The rollout is planned for imminently for most Exchange Online users. Microsoft have said they’re listening to customer feedback and will provide more control and visibility for administrators ahead of launch after there was some complaints that this would conflict with company policy in some cases.