Microsoft recently said it's working on an issue affecting Microsoft Outlook users who can't send emails, but most users won't be impacted — Microsoft determined that this bug only affects users with more than 500 shared folders in the primary mailbox. Due to limits placed on the email client, these users receive an 0x80040305 error from Outlook when sending emails.

In 2019, Microsoft addressed an issue with its shared folders by increasing the limit to 5,000. However, it did not adjust the setting for the primary mailbox, which went unnoticed until the number of users leveraging over 500 folders grew large enough to garner attention, thus eventually catching Microsoft's attention as complaints started flowing in.

At the time of writing, Microsoft hadn't provided an immediate solution. One of the workarounds for affected users is to reduce the 500 folder count to 450. This may not go over well with many users.

The second workaround is to keep all the mailbox folders collapsed instead of keeping them expanded. If for some reason that's not possible, Microsoft recommended avoiding certain online actions in Outlook such as using "View on Server" and "Click here to view more on Microsoft Exchange". Using "include older results" when searching also helps. After taking the required steps, the user will need to restart Outlook.

But Why Limit to 500 Sub-Folders?

This is a strange issue, but it exists by design, as the software-making giant intentionally placed this limit. The Outlook team explained that supporting more than 500 folders was prohibitively expensive as it affected performance when syncing with Microsoft Exchange, which evaluates changes whenever they are made. Outlook has problems with 'deep hierarchy' notifications, so setting a shallow limit allows the email client to function. At the time, having 500 subfolders seemed impossible to use, but many in key executive roles will surely have this type of setup for archival purposes.

So, to keep up with the times, Microsoft increased the limit to 5,000, which should be as quick as 500 shared folders. Since the primary inbox setting was overlooked, the team may simply enable the same workaround for the primary account.