In a Monday advisory, Microsoft warned Outlook.com users about issues they might encounter when sending emails containing attachments.
Outlook.com users impacted by this known issue are seeing "Error code 550 5.7.520 Message blocked" errors when trying to send emails.
"Some users may not be able to send emails that contain attachments from their Outlook.com mailbox," the company said.
Redmond suggested an alternative method that enables affected users to share files: uploading them to OneDrive and sharing the link with the intended recipients.
To implement this workaround, users must click the attachment button while composing their message and click OneDrive to add previously uploaded files or the 'Upload and share' button to add the attachment to their online storage drive.
Alternatively, those affected by this issue could copy the link of an Office or OneDrive file and paste it directly into their email.
Microsoft also provides step-by-step instructions to guide users through this process:
... deliver them! WTOutlook! 
