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.
Microsoft Outlook power users unable to send emails if they have more than 500 subfolders in their primary email account : Read more
Microsoft Outlook power users unable to send emails if they have more than 500 subfolders in their primary email account : Read more